Does your posted script work as expected?
It does not fill-in the field for me.
Is that code to get the time from a field or set the time for a field?
Is the field you are trying to update?
A time string for a computation is very limited or error prone unless it includes the date. There are many locations in the world that still observe Day Light Savings Time or Summer/Winter time and the time either moves forward one hour or falls back one hour at a certain date and time as prescribed by national or local law. So if one want to add 3 hours 1 hour before the time sift for the spring that would be 11:00pm adding 3 hours would result in a time value of 3:00 am the next day and not 2:00 am the next day. The hour between 1:00 am and 2:00 am does not exist for the local time zone.