char *s is the string buffer to put the text into. The function does not check bounds, so you must make sure that your buffer is large enough for the text of the date.
const char *format is a format string resembling a printf string in that it consists of text with certain %x substrings. You may use the constant HTTP_DATE_FMT to create date strings in the standard Internet format. For more information, see the documentation on the printf function for the runtime library of your compiler. Refer to Chapter 7, Time Formats for details on time formats.
const struct tm *t is a pointer to a calendar time (tm) structure, usually created by the function system_localtime or system_gmtime.