This is part of the RFC5545 specification, which is based on the ISO8601:

   Format Definition:  This value type is defined by the following
      notation:

       dur-value  = (["+"] / "-") "P" (dur-date / dur-time / dur-week)

       dur-date   = dur-day [dur-time]
       dur-time   = "T" (dur-hour / dur-minute / dur-second)
       dur-week   = 1*DIGIT "W"
       dur-hour   = 1*DIGIT "H" [dur-minute]
       dur-minute = 1*DIGIT "M" [dur-second]
       dur-second = 1*DIGIT "S"
       dur-day    = 1*DIGIT "D"

and I dont quite understand the dur-value formatting. I think that the / stand for an or right? That means that you should not be able to mix dur-date, dur-time and dur-week?

If thats the case and the chaining of dur-date = dur-day[dur-time] actually permits the use of dur-date and dur-time in the same duration value, but forbids that dur-week is mixed with anything else.

So this P1W1D duration value would be invalid?