http://icalendar.org/iCalendar-RFC-5545/3-3-6-duration.html - this is the reference to the document. It is somewhat self explanatory, but still a little bit hard to grasp. Maybe someone could explain what does the P in P15DT5H0M20S stand for? Also, maybe there is some other code-character that can be substituted for the P. If so, what are the other characters?
To understand the grammar, you might want to refer to the actual RFC 5545 section. Extract given below:
3.3.6. Duration Value Name: DURATION
Purpose: This value type is used to identify properties that contain a duration of time.
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"
from the standard, you can see that the letter P is an integral part of the grammar and that no other letter is considered.