How to add or subtract hours from GlideTime value (NOT GlideDateTime or GlideDate)?

I have a GlideTime (Time) type field in my table --- this is not GlideDateTime or GlideDate

Say for example, start_time

Now I want to add and subtract hours to the retrieved value (e.g. 5 hours)

That is, I want to do something like:

var actual_start_time = start_time + 5;

OR

var actual_start_time = start_time - 5;

Can someone please tell me, what is the most simplest way this can be done.