The General Utility Library provides a few time-related utility functions.

sleep(): Wait for a given amount of time (and be woken up from a different thread, if desired).

tic() & toc(): Measure elapsed time.

Furthermore, the library includes Howard Hinnant's date.h header, which extends C++14's <chrono> library with types and functionality for handling calendar dates. Most of this functionality has been standardized in C++20. Documentation is available on GitHub; just note that the GUL14 version lives in the namespace gul14::date.