Here is the only occasion where I must admit the official docs left me somewhat puzzled.
It shows examples such as 'now' + 20 which one could take as an indication on how to set a timer for current date time + 20 (hours, seconds ??)
My scenario is very simple - accept a file, wait a certain amount of time and then write the file.
So now I want to set this wait to 15 seconds -
This may seem pretty cryptic -
PT15S - PT are just constants -
so 3 days are P3D
3 hours PT3H
Check out Antony's blog posting for the why's etc.