Making time_t unsigned would break the ability to refer to times before 1970. Existing code that deals with 32-bit timestamps almost universally assumes that (

Making time_t unsigned would break the ability to refer to times before 1970. Existing code that deals with 32-bit timestamps almost universally assumes that (time_t)0 is 1970-01-01 00:00:00 UTC. Updating that code to guess a different epoch would be more work (with more inevitable bugs) than keeping a fixed epoch and using 64 bits. It's already 64 bits (and signed) on a lot of systems.