StartAt Time has been added and you can use a few different formats.
example url using the "t=" option: https://www.youtube.com/watch?v=Tw-WbgyfoYk&t=300
To start at 5min, you can use: t=300
or t=300s
or t=5m
To start at 4hours 19 mins, use: t=15540
or t=4h19m
or t=259m
so on..
oh yeah, I didn't think about the textpost body expandos. They're a little different. It's just an external link in this example. That link won't work as is. Updated thanks.
I was thinking of the t=time part of the url.