Unable to understand 'BYSETPOS' in the Docs

[marcoczen]

Guys,

With reference to

rrule("FREQ=MONTHLY;INTERVAL=1;BYSETPOS=-1;BYDAY=MO,TU,WE,TH,FR;BYMONTHDAY=13,14,15;COUNT=12")

I just cant seem to understand the purpose of 'bysetpos' ?
Anybody here can explain it ?

Thanks in advance ...

[boryn]

Hi @marcoczen!

With the example of "FREQ=MONTHLY;INTERVAL=1;BYSETPOS=-1;BYDAY=MO,TU,WE,TH,FR;BYMONTHDAY=26,27,28,29,30,31;COUNT=12"
you have a set of occurences falling at 26-31 of the month and only on MO,TU,WE,TH,FR.

BYSETPOS is not a rule by itself, actually it limits the rule. It means take the n-th instance from the calculated instances corresponding to the rest of the rule. So in our case it means take the last day at the end of the month of the working days.

Hope it clarifies :)

[marcoczen]

Hi @boryn ,

BYSETPOS is not a rule by itself, actually it limits the rule.

Noted ...

It means take the n-th instance from the calculated instances corresponding to the rest of the rule

-1 ? nth instance ... but its not 2 or 3 or 1 but -1 ?
Also, if i want the last of an occurence, its -1 ? Cant it be +366 or +365 ?

Also bysetpos works on byday or bymonthday ? or both ?

Sorry if i am a dumbass....

[marcoczen]

Background info for the next noob ...

From https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.5

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.

[marcoczen]

Update ... Without bysetpos set to -1 or 1, we could have multiple dates in the same month ...