Fix #9974, aa5a8fe: strftime buffer maxsize is off by one

strftime() return value doesn't count NULL character, but buffer size must include it.

Codechange: use thread safe time functions

Functions like localtime, gmtime and asctime are not thread safe as they (might) reuse the same buffer. So use the safer _s/_r variant for localtime and gmtime, and use strftime in favour of asctime.