Why is the kdb+ epoch date 2000.01.01?

Question

I am new to kdb+ and I was wondering why the epoch date of 2000.01.01 for kdb is different to that of unix (1970.01.01).

Does the difference affect any interactions with the operating system or other languages?

score 3 · Accepted Answer · answered Nov 22 '17 at 17:19

KDB+ uses a different epoch because it follows a different standard. KDB+ follows the J2000 international standard, which is based on the Julian year.

UNIX uses POSIX time, which was initially a 32 bit unsigned integer. Because the time was calculated to 60ths of a second, the 32 bit integer would only work for about 829 days, so a recent date had to be chosen.

The first edition Unix Programmer's Manual dated November 3, 1971 defines the Unix time as "the time since 00:00:00, Jan. 1, 1971, measured in sixtieths of a second"

This difference can cause issues if you don't make sure to convert to one standard before piping the chosen epoch time into applications.

Jacob, it was revised **later on**. Initially, it did say 1971. [StackOverflow answer](https://stackoverflow.com/questions/11573024/when-was-unix-epoch-time-revised-from-1971-to-current-1970-value#11597932) — DeathCamel57, Nov 27 '17 at 03:18

score 3 · Answer 2 · answered Nov 22 '17 at 17:22

Issues regarding interactions with system/other languages should be able to be dealt with by the fact KDB can parse UNIX epoch timestamps

from http://code.kx.com/q/ref/casting/#tok

Parsing Unix timestamps (from seconds since Unix epoch), string with 9…11 digits:
q)"P"$"10129708800"
2290.12.31D00:00:00.000000000
q)"P"$"00000000000"
1970.01.01D00:00:00.000000000

score 0 · Answer 3 · answered Nov 22 '17 at 17:27

Kdb+ is available on many different operating systems, currently Windows, Linux-x86, Linux-ARM and OSX are available to download from kx, with solaris previous available.

From the page about system time on Wikipedia we can see that various operating systems use differing epoch dates and ranges. Considering two OS that kdb+ supports we can see that they have differing epoch ranges:

OS              Epoch or range
-------------------------------------------------
Unix/Posix      1 January 1970 to 19 January 2038
Windows         1 January 1601 to AD 30,828

Using either the linux or windows epoch would mean that the other did not match up anyway. Further reading on that page also shows that many other languages also use their own distinct epoch dates and ranges.

In short, there is no reason a language needs to use the epoch time of the OS it is running on.

Why is the kdb+ epoch date 2000.01.01?

3 Answers3