Select Interval in plpgsql

I can't seem to figure out how to accomplish this task in a plpgsql function:

I have a timestamp variable 'apt_time' and a varchar variable 'time_offset'. The time_offset variable usually looks like this: '-2 hours'. What I want to do is add the time_offset to the timestamp. So I have tried:

SELECT apt_time + interval time_offset INTO Adjusted_Time;

Where Adjusted_Time is the variable to store the result.

But I keep getting errors. I'm not sure where the problem is, as I obviously have not set up the field or variable types properly, or have structured the SELECT improperly.

This statement works just fine:

SELECT apt_time + interval '-2 hours' INTO Adjusted_Time;

but when I try to use a variable with the interval, I can't get it to work.

Any help would be appreciated.

Thanks,
Derrick