Convert time in seconds to HTTP header format
This function converts the time (specified as the number of seconds from 00:00:00 UTC, January 1, 1970) to the appropriate format for an HTTP header or log file. The time and date is returned with a four digit year, for example: "Sun, 06 Nov 1994 08:49:37 GMT"
You can use the ns_time function to get the current time in seconds.