time-portable.hs

-- | Converts a 'ClockTime' into an HTTP timestamp.
formatHttpTime :: UTCTime -> ByteString
formatHttpTime = fromStr . formatTime defaultTimeLocale "%a, %d %b %Y %X GMT"

-- | Converts an HTTP timestamp into a 'UTCTime'.
parseHttpTime :: ByteString -> Maybe UTCTime
parseHttpTime s' =
    parseTime defaultTimeLocale "%a, %d %b %Y %H:%M:%S GMT" s
  where
    s = toStr s'

time-unix.hs

-- | Converts a 'CTime' into an HTTP timestamp.
formatHttpTime :: CTime -> IO ByteString
formatHttpTime t = allocaBytes 40 $ \ptr -> do
    c_format_http_time t ptr
    S.packCString ptr


------------------------------------------------------------------------------
-- | Converts an HTTP timestamp into a 'CTime'.
parseHttpTime :: ByteString -> IO CTime
parseHttpTime s = S.useAsCString s $ \ptr ->
    c_parse_http_time ptr

timefuncs.c

#define _XOPEN_SOURCE
#include <time.h>
#include <locale.h>


void set_c_locale() {
    setlocale(LC_TIME, "C");
}


time_t c_parse_http_time(char* s) {
    struct tm dest;
    strptime(s, "%a, %d %b %Y %H:%M:%S GMT", &dest);
    return mktime(&dest);
}

void c_format_http_time(time_t src, char* dest) {
    struct tm t;
    gmtime_r(&src, &t);
    strftime(dest, 40, "%a, %d %b %Y %H:%M:%S GMT", &t);
}