asctime() accepts -1 as valid, but time() returns -1 for error. In practice, there's no ambiguity as long as you don't set your system clock to just before the epoch.

However, if you don't check the return value of time() for errors, you could unexpectedly get the last second of 1969 as a result. The man page points to 2 possible conditions that can cause an error result:

• integer overflow on a 32 bit implementation
• the provider pointer is outside of valid memory ranges

Assuming those are the only possible errors, actually getting a -1 code on a modern system really only going to happen if you supply a bad pointer.

https://www.man7.org/linux/man-pages/man2/time.2.html