Write a date in ascii as in the http protocol
#include <fmt.h>
size_t fmt_httpdate(char *dest,time_t source);
fmt_httpdate writes a date in ASCII representation as the HTTP protocol defines it: "Sun, 06 Nov 1994 08:49:37 GMT".
fmt_httpdate does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_httpdate returns the number of bytes it would have written.
The output of fmt_httpdate is 29 until Jan 1st 10000.