time.h is not a typical "Arduino library". time.h is a part of the esp core.
For the ESP8266 there is an IDE example. you will find it in
Examples/ESP8266/NTP-TZ-DST
but imho my stripped down versions are simpler to read.
p.s.: the time.h file is in my installation here:
but imho there is "nothing" to see there for someone like me. I just use the default API of time.h and the members of the structure tm.