On Wed, Sep 29, 2010 at 8:56 AM, Dave Hart <[email protected]> wrote: > On Tue, Sep 28, 2010 at 12:40 UTC, Matt Davis <[email protected]> wrote: >> I am having some trouble converting a POSIX UTC timestamp into a NTP >> timestamp. >> The former being from the epoch starting January 1, 1970 at 12AM and the >> latter >> being that of January 1 1900 12AM. >> >> In summary my equation is this: >> >> /* Seconds between 1970 and 1900 (with 17 leap years) */ >> uint64_t epoch_difference = (365 + 17) * 24 * 60 * 60; >> >> uint64_t ntp_timestamp = (time(NULL) - epoch_difference) << 32; >> >> Unfortunately, when I interpret this value, I keep getting values in 2036. I >> know my version of time() is from the 1970 epoch. So I am a tad confused >> here. > > As was said on irc.freenode.net #ntp in response to your question > there, you are subtracting when you should be adding. There are more > seconds between now and 1900 than between now and 1970. The NTP > reference implementation has this code ready for you to use verbatim, > liberally licensed. See include/unixtime.h and libntp/systime.c, > particularly get_systime(). Note in particular that the reference > implementation disagrees with your code on the value of > epoch_difference (referred to as JAN_1970 in the NTP code): > > #define JAN_1970 0x83aa7e80 /* 2208988800 1970 - 1900 in seconds > */
Dave, I do recall the addition mentioned in irc, but I was trying to get a sanity check, as I was not having any luck either way (with addition). I appreciate your information. I was not aware of the macro you specified. Thanks! -Matt _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
