value encode_date = fun (year, mon, day, hour, min, sec) ->
    let (tm, _) = mktime {tm_sec=sec; tm_min=min; tm_hour=hour; 
                          tm_mday=day; tm_mon=mon-1; tm_year=year-1900; 
                          tm_wday=0; tm_yday=0; tm_isdst=False} 
    in tm /. 86400.0