Time&Date Conversion

Looking for examples on the following.
If we have a display Date, Time, and time-zone,
such as "1999-07-21 14:20:00 PST", how can
one convert to a GMT(UTC) time_t value.
Of course, I would like to go the opposite
direction using UTC time_t and a given
Timezone to create the display"

Anything in C or C++ would be fine. I currently
have my own solution using the c-runtime libs,
but I would like to know of any methods
for this scenario.

Thanks,