A timestamp isn't just hours and minutes, it also contains the year and the date. To be more exact it's the seconds since 1970. I think there is no way
with sourcemod to calculate a timestamp based on a datetime.
So you have to compare the hours and minutes as numbers and calculate the time difference yourself.
Time difference = (currentHours*3600+currentMinutes*60)-(keyValueHours*3600+keyValueMinutes*60)
KeyValuesTimestamp = GetTime() - Time difference
I'm not sure if that's what you want, but I think it might give you a hint how to start
