It isn't that simple it will break binary compatibility. 64 bit systems should be fine as but older 32 bit will be a problem and there is a lot firmware, embedded systems that won't be updated on time and that's where most of the problems will show up.
typedef long time_t;
to
typedef long long time_t;
Thereby doubling the amount of space for time_t. Future applications written in c should have this fix or something like it.