# [ntp:questions] POSIX Timestamp to NTP Timestamp

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Wed Sep 29 07:00:10 UTC 2010

```Dave Hart wrote:
> On Tue, Sep 28, 2010 at 12:40 UTC, Matt Davis<mattdavis9 at gmail.com>  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;

As noted, this is just wrong.

Even if you fix it as suggested ((365*70+17)*86400) you still have a big
problem:

Even though you are assigning the result to a 64-bit unsigned int, the
actual calculation will probably take place within 32-bit signed
arithmetic, in which case it will overflow into a negative value.

You have to make sure that the constant calculation is handled with
64-bit int or at least 32-bit unsigned:

uint64_t epoch_difference = ((uint64_t)365*70 + 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. Â I
>> know my version of time() is from the 1970 epoch. Â 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 */

Notice also that this 32-bit value has the sign bit set, i.e. it must be
handled as uint32_t or wider.

Terje

--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

```