Is this not supported? I seem to be completely unable to store a duration in a variable unless I state that duration explicitly in the skript;

Code (Skript):

1. command /duration <string>:
2.     trigger:
3.         set {_old} to now
4.         wait 5 seconds
5.         set {_time} to arg 1 parsed as time period
6.         message "%difference between {_time} and difference between {_old} and now%"

Try this with 3 seconds as your first argument; It should return "2 seconds"
I've also attempted to parse it as duration and timeperiod
It seems that it's unsupported, but I figured I'd check here to see if it's simply me not knowing about a syntax or something.

For reference, the following skript does work as expected, which confirms that variables can definitely store durations:

Code (Skript):

1. command /durationfixed:
2.     trigger:
3.         set {_old} to now
4.         wait 5 seconds
5.         set {_time} to 3 seconds
6.         message "%difference between {_time} and difference between {_old} and now%"

Would greatly appreciate any help!