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.