By default, the Repetitive property is "", which indicates that the event is not a repetitive event. If You specify a not empty and valid formula for the Repetitive property, the time part of the Start and End properties determines the time to start and end the repetitive event. The date part is determined by the Repetitive expression. You can use the KnownProperty( exEventRepetitive) property to determine whether the current event is repetitive or not. The property returns True, if the Repetitive property is not empty and valid, and False, if the Repetitive property is empty or not valid. 

Starting with the version 12.1, the control supports two ways of representing a Repetitive/Recurrence event:

• Value format, when using the value keyword. For instance, "weekday(value) = 1", the event occurs every Monday
• ICalendar format, as described in RFC 5545. For instance, "FREQ=WEEKLY;BYDAY=MO", the event occurs every Monday

The FREQ property determines whether the Repetitive property uses the Value or ICalendar format. In other words, if the Repetitive property contains the FREQ keyword, the ICalendar format is using, else the Value format.

Here's a few samples of Repetitive expressions:

• "0", no occurrence
• "1", the event occurs every day
• "weekday(value) = 1", the event occurs every Monday
• "weekday(value) in (1,2) and month(value) = 6", the event occurs every Monday and Tuesday, on June only. 
• "value in (#6/8/2012#,#6/11/2012#,#6/20/2012#)", the event occurs on 6/8/2012, 6/11/2012 and 6/20/2012
• "value >= #6/1/2012# and ( (value - #6/1/2012#) mod 5 = 0)", the event starts on 6/1/2012, and shows up every 5 days
• "(value >= (0:=#6/1/2012#)) and ( (value - =:0) mod (1:=5) = 0) and (value-=:0) < (3*=:1)", the event starts on 6/1/2012, occurs every 5 days, for 3 times. You can change 6/1/2012 with your date to indicates the starting date, changes 5 to indicate the n-occurrence and change 3 to indicate the m-times, so the event is shown every n-days for m-times.
• "not(month(value) in (3,4,5)) ? 0 : ( floor(value)=(2:=floor(date(dateS('3/1/' + year(value)) + ((1:=(((255 - 11 * (year(value) mod 19)) - 21) mod 30) + 21) + (=:1 > 48 ? -1 : 0) + 6 - ((year(value) + int(year(value) / 4)) + =:1 + (=:1 > 48 ? -1 : 0) + 1) mod 7)))))", indicates the Easter- Sunday, so the event shows every year on Easter sunday.

The Repetitive property supports the value keyword which indicates the date being queried, and the following predefined operators and functions.

The supported binary arithmetic operators are:

• * ( multiplicity operator ), priority 5
• / ( divide operator ), priority 5
• mod ( reminder operator ), priority 5
• + ( addition operator ), priority 4 ( concatenates two strings, if one of the operands is of string type )
• - ( subtraction operator ), priority 4

The supported unary boolean operators are:

• not ( not operator ), priority 3 ( high priority )

The supported binary boolean operators are:

• or ( or operator ), priority 2
• and ( or operator ), priority 1

The supported binary boolean operators, all these with the same priority 0, are :

• < ( less operator )
• <= ( less or equal operator )
• = ( equal operator )
• != ( not equal operator )
• >= ( greater or equal operator )
• > ( greater operator )

The supported ternary operators, all these with the same priority 0, are :

• ? ( Immediate If operator ), returns and executes one of two expressions, depending on the evaluation of an expression. The syntax for is

"expression ? true_part : false_part"

, while it executes and returns the true_part if the expression is true, else it executes and returns the false_part. For instance, the "%0 = 1 ? 'One' : (%0 = 2 ? 'Two' : 'not found')" returns 'One' if the value is 1, 'Two' if the value is 2, and 'not found' for any other value. A n-ary equivalent operation is the case() statement, which is available in newer versions of the component.

The supported n-ary operators are (with priority 5):

• array (at operator), returns the element from an array giving its index ( 0 base ). The array operator returns empty if the element is found, else the associated element in the collection if it is found. The syntax for array operator is

"expression array (c1,c2,c3,...cn)"

, where the c1, c2, ... are constant elements. The constant elements could be numeric, date or string expressions. For instance the "month(value)-1 array ('J','F','M','A','M','Jun','J','A','S','O','N','D')" is equivalent with "month(value)-1 case (default:''; 0:'J';1:'F';2:'M';3:'A';4:'M';5:'Jun';6:'J';7:'A';8:'S';9:'O';10:'N';11:'D')".

• in (include operator), specifies whether an element is found in a set of constant elements. The in operator returns -1 ( True ) if the element is found, else 0 (false) is retrieved. The syntax for in operator is

"expression in (c1,c2,c3,...cn)"

, where the c1, c2, ... are constant elements. The constant elements could be numeric, date or string expressions. For instance the "value in (11,22,33,44,13)" is equivalent with "(expression = 11) or (expression = 22) or (expression = 33) or (expression = 44) or (expression = 13)". The in operator is not a time consuming as the equivalent or version is, so when you have large number of constant elements it is recommended using the in operator. Shortly, if the collection of elements has 1000 elements the in operator could take up to 8 operations in order to find if an element fits the set, else if the or statement is used, it could take up to 1000 operations to check, so by far, the in operator could save time on finding elements within a collection.

• switch (switch operator), returns the value being found in the collection, or a predefined value if the element is not found (default). The syntax for switch operator is

"expression switch (default,c1,c2,c3,...,cn)"

, where the c1, c2, ... are constant elements, and the default is a constant element being returned when the element is not found in the collection. The constant elements could be numeric, date or string expressions. The equivalent syntax is "%0 = c 1 ? c 1 : ( %0 = c 2 ? c 2 : ( ... ? . : default) )". The switch operator is very similar with the in operator excepts that the first element in the switch is always returned by the statement if the element is not found, while the returned value is the value itself instead -1. For instance, the "%0 switch ('not found',1,4,7,9,11)" gets 1, 4, 7, 9 or 11, or 'not found' for any other value. As the in operator the switch operator uses binary searches for fitting the element, so it is quicker that iif (immediate if operator) alterative.

• case() (case operator) returns and executes one of n expressions, depending on the evaluation of the expression ( IIF - immediate IF operator is a binary case() operator ). The syntax for case() operator is:

"expression case ([default : default_expression ; ] c1 : expression1 ; c2 : expression2 ; c3 : expression3 ;....)"

If the default part is missing, the case() operator returns the value of the expression if it is not found in the collection of cases ( c1, c2, ...). For instance, if the value of expression is not any of c1, c2, .... the default_expression is executed and returned. If the value of the expression is c1, then the case() operator executes and returns the expression1. The default, c1, c2, c3, ... must be constant elements as numbers, dates or strings. For instance, the "date(shortdate(value)) case (default:0 ; #1/1/2002#:1 ; #2/1/2002#:1; #4/1/2002#:1; #5/1/2002#:1)" indicates that only #1/1/2002#, #2/1/2002#, #4/1/2002# and #5/1/2002# dates returns 1, since the others returns 0. For instance the following sample specifies the hour being non-working for specified dates: "date(shortdate(value)) case(default:0;#4/1/2009# : hour(value) >= 6 and hour(value) <= 12 ; #4/5/2009# : hour(value) >= 7 and hour(value) <= 10 or hour(value) in(15,16,18,22); #5/1/2009# : hour(value) <= 8)" statement indicates the working hours for dates as follows:

• • #4/1/2009#, from hours 06:00 AM to 12:00 PM
  • #4/5/2009#, from hours 07:00 AM to 10:00 AM and hours 03:00PM, 04:00PM, 06:00PM and 10:00PM
  • #5/1/2009#, from hours 12:00 AM to 08:00 AM

The in, switch and case() use binary search to look for elements so they are faster then using iif and or expressions.

Obviously, the priority of the operations inside the expression is determined by ( ) parenthesis and the priority for each operator.

The supported conversion unary operators are:

• type (unary operator) retrieves the type of the object. For instance type(%0) = 8 specifies the cells that contains string values.

  Here's few predefined types:

  • 0 - empty ( not initialized )
  • 1 - null
  • 2 - short
  • 3 - long
  • 4 - float
  • 5 - double
  • 6 - currency
  • 7 - date
  • 8 - string
  • 9 - object
  • 10 - error
  • 11 - boolean
  • 12 - variant
  • 13 - any
  • 14 - decimal
  • 16 - char
  • 17 - byte
  • 18 - unsigned short
  • 19 - unsigned long
  • 20 - long on 64 bits
  • 21 - unsigned long on 64 bites
• str (unary operator) converts the expression to a string
• dbl (unary operator) converts the expression to a number
• date (unary operator) converts the expression to a date, based on your regional settings. The date(``) returns now ( date + time ), and int(date(``)) gets the today date ( with no time including )
• dateS (unary operator) converts the string expression to a date using the format MM/DD/YYYY HH:MM:SS.

Other known operators for numbers are:

• int (unary operator) retrieves the integer part of the number
• round (unary operator) rounds the number ie 1.2 gets 1, since 1.8 gets 2
• floor (unary operator) returns the largest number with no fraction part that is not greater than the value of its argument
• abs (unary operator) retrieves the absolute part of the number ie -1 gets 1, 2 gets 2
• value format 'flags' (binary operator) formats the value with specified flags. If flags is empty, the number is displayed as shown in the field "Number" in the "Regional and Language Options" from the Control Panel. For instance the 1000 format '' displays 1,000.00 for English format, while 1.000,00 is displayed for German format. 1000 format '2|.|3|,' will always displays 1,000.00 no matter of settings in the control panel. If formatting the number fails for some invalid parameter, the value is displayed with no formatting.

  The ' flags' for format operator is a list of values separated by | character such as 'NumDigits|DecimalSep|Grouping|ThousandSep|NegativeOrder|LeadingZero' with the following meanings:

  • NumDigits - specifies the number of fractional digits, If the flag is missing, the field "No. of digits after decimal" from "Regional and Language Options" is using.
  • DecimalSep - specifies the decimal separator. If the flag is missing, the field "Decimal symbol" from "Regional and Language Options" is using.
  • Grouping - indicates the number of digits in each group of numbers to the left of the decimal separator. Values in the range 0 through 9 and 32 are valid. The most significant grouping digit indicates the number of digits in the least significant group immediately to the left of the decimal separator. Each subsequent grouping digit indicates the next significant group of digits to the left of the previous group. If the last value supplied is not 0, the remaining groups repeat the last group. Typical examples of settings for this member are: 0 to group digits as in 123456789.00; 3 to group digits as in 123,456,789.00; and 32 to group digits as in 12,34,56,789.00. If the flag is missing, the field "Digit grouping" from "Regional and Language Options" indicates the grouping flag.
  • ThousandSep - specifies the thousand separator. If the flag is missing, the field "Digit grouping symbol" from "Regional and Language Options" is using.
  • NegativeOrder - indicates the negative number mode. If the flag is missing, the field "Negative number format" from "Regional and Language Options" is using. The valid values are 0, 1, 2, 3 and 4 with the following meanings:
    • 0 - Left parenthesis, number, right parenthesis; for example, (1.1)
    • 1 - Negative sign, number; for example, -1.1
    • 2 - Negative sign, space, number; for example, - 1.1
    • 3 - Number, negative sign; for example, 1.1-
    • 4 - Number, space, negative sign; for example, 1.1 -
  • LeadingZero - indicates if leading zeros should be used in decimal fields. If the flag is missing, the field "Display leading zeros" from "Regional and Language Options" is using. The valid values are 0, 1

Other known operators for strings are:

• len (unary operator) retrieves the number of characters in the string
• lower (unary operator) returns a string expression in lowercase letters
• upper (unary operator) returns a string expression in uppercase letters
• proper (unary operator) returns from a character expression a string capitalized as appropriate for proper names
• ltrim (unary operator) removes spaces on the left side of a string
• rtrim (unary operator) removes spaces on the right side of a string
• trim (unary operator) removes spaces on both sides of a string
• startwith (binary operator) specifies whether a string starts with specified string
• endwith (binary operator) specifies whether a string ends with specified string
• contains (binary operator) specifies whether a string contains another specified string
• left (binary operator) retrieves the left part of the string
• right (binary operator) retrieves the right part of the string
• a mid b (binary operator) retrieves the middle part of the string a starting from b ( 1 means first position, and so on )
• a count b (binary operator) retrieves the number of occurrences of the b in a
• a replace b with c (double binary operator) replaces in a the b with c, and gets the result.
• a split b, splits the a using the separator b, and returns an array. For instance, the "weekday(value) array 'Sun Mon Thu Wed Thu Fri Sat' split ' '" gets the weekday as string. This operator can be used with the array

Other known operators for dates are:

• time (unary operator) retrieves the time of the date in string format, as specified in the control's panel.
• timeF (unary operator) retrieves the time of the date in string format, as "HH:MM:SS". For instance the timeF(1:23 PM) returns "13:23:00"
• shortdate (unary operator) formats a date as a date string using the short date format, as specified in the control's panel.
• shortdateF (unary operator) formats a date as a date string using the "MM/DD/YYYY" format. For instance the shortdateF(December 31, 1971 11:00 AM) returns "12/31/1971".
• dateF (unary operator) converts the date expression to a string expression in "MM/DD/YYYY HH:MM:SS" format.
• longdate (unary operator) formats a date as a date string using the long date format, as specified in the control's panel.
• year (unary operator) retrieves the year of the date (100,...,9999)
• month (unary operator) retrieves the month of the date ( 1, 2,...,12 )
• day (unary operator) retrieves the day of the date ( 1, 2,...,31 )
• yearday (unary operator) retrieves the number of the day in the year, or the days since January 1st ( 0, 1,...,365 )
• weekday (unary operator) retrieves the number of days since Sunday ( 0 - Sunday, 1 - Monday,..., 6 - Saturday )
• hour (unary operator) retrieves the hour of the date ( 0, 1, ..., 23 )
• min (unary operator) retrieves the minute of the date ( 0, 1, ..., 59 )
• sec (unary operator) retrieves the second of the date ( 0, 1, ..., 59 )

The BNF syntax for ICalendar format is:

recur = recur-rule-part *( ";" recur-rule-part )
	;
	; The rule parts are not ordered in any
	; particular sequence.
	;
	; The FREQ rule part is REQUIRED,
	; but MUST NOT occur more than once.
	;
	; The UNTIL or COUNT rule parts are OPTIONAL,
	; but they MUST NOT occur in the same 'recur'.
	;
	; The other rule parts are OPTIONAL,
	; but MUST NOT occur more than once.

recur-rule-part = ( "FREQ" "=" freq )
				/ ( "UNTIL" "=" enddate )
				/ ( "COUNT" "=" 1*DIGIT )
				/ ( "INTERVAL" "=" 1*DIGIT )
				/ ( "BYSECOND" "=" byseclist )
				/ ( "BYMINUTE" "=" byminlist )
				/ ( "BYHOUR" "=" byhrlist )
				/ ( "BYDAY" "=" bywdaylist )
				/ ( "BYMONTHDAY" "=" bymodaylist )
				/ ( "BYYEARDAY" "=" byyrdaylist )
				/ ( "BYWEEKNO" "=" bywknolist )
				/ ( "BYMONTH" "=" bymolist )
				/ ( "BYSETPOS" "=" bysplist )
				/ ( "WKST" "=" weekday )

freq        = "SECONDLY" / "MINUTELY" / "HOURLY" / "DAILY"
			/ "WEEKLY" / "MONTHLY" / "YEARLY"
enddate     = date / date-time
byseclist   = ( seconds *("," seconds) )
seconds     = 1*2DIGIT       ;0 to 60
byminlist   = ( minutes *("," minutes) )
minutes     = 1*2DIGIT       ;0 to 59
byhrlist    = ( hour *("," hour) )
hour        = 1*2DIGIT       ;0 to 23
bywdaylist  = ( weekdaynum *("," weekdaynum) )
weekdaynum  = [[plus / minus] ordwk] weekday
plus        = "+"
minus       = "-"
ordwk       = 1*2DIGIT       ;1 to 53
weekday     = "SU" / "MO" / "TU" / "WE" / "TH" / "FR" / "SA" ;Corresponding to SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, and SATURDAY days of the week.
bymodaylist = ( monthdaynum *("," monthdaynum) )
monthdaynum = [plus / minus] ordmoday
ordmoday    = 1*2DIGIT       ;1 to 31
byyrdaylist = ( yeardaynum *("," yeardaynum) )
yeardaynum  = [plus / minus] ordyrday
ordyrday    = 1*3DIGIT      ;1 to 366
bywknolist  = ( weeknum *("," weeknum) )
weeknum     = [plus / minus] ordwk
bymolist    = ( monthnum *("," monthnum) )
monthnum    = 1*2DIGIT       ;1 to 12
bysplist    = ( setposday *("," setposday) )
setposday   = yeardaynum

This value type is a structured value consisting of a list of one or more recurrence grammar parts.  Each rule part is defined by a NAME=VALUE pair.  The rule parts are separated from each other by the SEMICOLON character.  The rule parts are not
ordered in any particular sequence.  Individual rule parts MUST only be specified once.  Compliant applications MUST accept rule parts ordered in any sequence, but to ensure backward compatibility with applications that pre-date this revision of  iCalendar the FREQ rule part MUST be the first rule part specified in a RECUR value.

The FREQ rule part identifies the type of recurrence rule. This rule part MUST be specified in the recurrence rule. Valid values include SECONDLY, to specify repeating events based on an interval of a second or more; MINUTELY, to specify repeating events based on an interval of a minute or more; HOURLY, to specify repeating events based on an interval of an hour or more; DAILY, to specify repeating events based on an interval of a day or more; WEEKLY, to specify repeating events based on an interval of a week or more; MONTHLY, to specify repeating events based on an interval of a month or more; and YEARLY, to specify repeating events based on an interval of a year or more.

The INTERVAL rule part contains a positive integer representing at which intervals the recurrence rule repeats. The default value is "1", meaning every second for a SECONDLY rule, every minute for a MINUTELY rule, every hour for an HOURLY rule, every day for a DAILY rule, every week for a WEEKLY rule, every month for a MONTHLY rule, and every year for a YEARLY rule. For example, within a DAILY rule, a value of "8" means every eight days.

The UNTIL rule part defines a DATE or DATE-TIME value that bounds the recurrence rule in an inclusive manner. If the value specified by UNTIL is synchronized with the specified recurrence, this DATE or DATE-TIME becomes the last instance of the recurrence. The value of the UNTIL rule part MUST have the same value type as the "DTSTART" property. Furthermore, if the "DTSTART" property is specified as a date with local time, then the UNTIL rule part MUST also be specified as a date with local time. If the "DTSTART" property is specified as a date with UTC time or a date with local time and time zone reference, then the UNTIL rule part MUST be specified as a date with UTC time. In the case of the "STANDARD" and "DAYLIGHT" sub-components the UNTIL rule part MUST always be specified as a date with UTC time. If specified as a DATE-TIME value, then it MUST be specified in a UTC time format. If not present, and the COUNT rule part is also not present, the "RRULE" is considered to repeat forever.

The COUNT rule part defines the number of occurrences at which to range-bound the recurrence. The "DTSTART" property value always counts as the first occurrence.

The BYSECOND rule part specifies a COMMA-separated list of seconds within a minute. Valid values are 0 to 60. The BYMINUTE rule part specifies a COMMA-separated list of minutes within an hour. Valid values are 0 to 59. The BYHOUR rule part specifies a COMMA- separated list of hours of the day. Valid values are 0 to 23. The BYSECOND, BYMINUTE and BYHOUR rule parts MUST NOT be specified when the associated "DTSTART" property has a DATE value type. These rule parts MUST be ignored in RECUR value that violate the above requirement (e.g., generated by applications that pre-date this revision of iCalendar).

The BYDAY rule part specifies a COMMA-separated list of days of the week; SU indicates Sunday; MO indicates Monday; TU indicates Tuesday; WE indicates Wednesday; TH indicates Thursday; FR indicates Friday; and SA indicates Saturday. Each BYDAY value can also be preceded by a positive (+n) or negative (-n) integer. If present, this indicates the nth occurrence of a specific day within the MONTHLY or YEARLY "RRULE".
For example, within a MONTHLY rule, +1MO (or simply 1MO) represents the first Monday within the month, whereas -1MO represents the last Monday of the month. The numeric value in a BYDAY rule part with the FREQ rule part set to YEARLY corresponds to an offset within the month when the BYMONTH rule part is present, and corresponds to an offset within the year when the BYWEEKNO or BYMONTH rule parts are present. If an integer modifier is not present, it means all days of this type within the specified frequency. For example, within a MONTHLY rule, MO represents all Mondays within the month. The BYDAY rule part MUST NOT be specified with a numeric value when the FREQ rule part is not set to MONTHLY or YEARLY. Furthermore, the BYDAY rule part MUST NOT be specified with a numeric value with the FREQ rule part set to YEARLY when the BYWEEKNO rule part is specified.

The BYMONTHDAY rule part specifies a COMMA-separated list of days of the month. Valid values are 1 to 31 or -31 to -1. For example, -10 represents the tenth to the last day of the month. The BYMONTHDAY rule part MUST NOT be specified when the FREQ rule part is set to WEEKLY.

The BYYEARDAY rule part specifies a COMMA-separated list of days of the year. Valid values are 1 to 366 or -366 to -1. For example, -1 represents the last day of the year (December 31st) and -306 represents the 306th to the last day of the year (March 1st). The BYYEARDAY rule part MUST NOT be specified when the FREQ rule part is set to DAILY, WEEKLY, or MONTHLY.

The BYWEEKNO rule part specifies a COMMA-separated list of ordinals specifying weeks of the year. Valid values are 1 to 53 or -53 to -1. This corresponds to weeks according to week numbering as defined in [ISO.8601.2004]. A week is defined as a seven day period, starting on the day of the week defined to be the week start (see WKST). Week number one of the calendar year is the first week that contains at least four (4) days in that calendar year. This rule part MUST NOT be used when the FREQ rule part is set to anything other than YEARLY. For example, 3 represents the third week of the year.

Note: Assuming a Monday week start, week 53 can only occur when Thursday is January 1 or if it is a leap year and Wednesday is January 1.

The BYMONTH rule part specifies a COMMA-separated list of months of the year. Valid values are 1 to 12.

The WKST rule part specifies the day on which the workweek starts. Valid values are MO, TU, WE, TH, FR, SA, and SU. This is significant when a WEEKLY "RRULE" has an interval greater than 1, and a BYDAY rule part is specified. This is also significant when in a YEARLY "RRULE" when a BYWEEKNO rule part is specified. The default value is MO.

The BYSETPOS rule part specifies a COMMA-separated list of values that corresponds to the nth occurrence within the set of recurrence instances specified by the rule. BYSETPOS operates on a set of recurrence instances in one interval of the recurrence rule. For example, in a WEEKLY rule, the interval would be one week A set of recurrence instances starts at the beginning of the interval defined by the FREQ rule part. Valid values are 1 to 366 or -366 to -1. It MUST only be used in conjunction with another BYxxx rule part. For example "the last work day of the month" could be represented as:

FREQ=MONTHLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=-1

Each BYSETPOS value can include a positive (+n) or negative (-n) integer. If present, this indicates the nth occurrence of the specific occurrence within the set of occurrences specified by the rule.

Recurrence rules may generate recurrence instances with an invalid date (e.g., February 30) or nonexistent local time (e.g., 1:30 AM on a day where the local time is moved forward by an hour at 1:00 AM). Such recurrence instances MUST be ignored and MUST NOT be counted as part of the recurrence set. Information, not contained in the rule, necessary to determine the various recurrence instance start time and dates are derived from the Start Time ("DTSTART") component attribute. For example, "FREQ=YEARLY;BYMONTH=1" doesn't specify a specific day within the month or a time. This information would be the same as what is specified for "DTSTART".