[time-nuts] MJD 60000 this week

[Tom Van Baak]

Here's a nerdy timing post...

It's now Monday in UTC-land; the current time is:

2023-02-20 00:00:00 UTC (DOY = 51, Mon, STOD = 0) = JD 2459995.50000 = 
MJD 59995.000000
Sun Feb 19 16:00:00 2023 (UTC-8 Pacific Standard Time)

This means in 5 days it will be MJD 60000. There likely won't be any 
trouble but even I have some scripts that do "sanity checks" on MJD 
expecting the first digit to be 5 or maybe 4. They will be fixed before 
they fail at the end of the week.

Humans prefer calendars (year, month, day) and clock faces (hours, 
minutes, seconds) in spite of the awkward variable length of months and 
years or Babylonian base 24 or 60 arithmetic for time of day. In 
contrast, most h/w and s/w prefers to work with some form of integer 
linear time because it makes the math so much easier.

Keeping track of linear time with JD (Julian Date) is very common in 
astronomy. Keeping track of linear time with MJD (Modified Julian Date) 
is very common in time & frequency metrology, including satellites such 
as GPS. [1] You've probably read papers about time scales, atomic 
clocks, UTC(k), GPS performance, and stuff like that. The x-axis is very 
commonly MJD.

All linear time scales have an origin date (aka epoch). Here are some 
examples:

//Â Â  01-Jan-1601 (JD 2305813.5) is WNT (Windows NT) epoch
//   17-Nov-1858 (JD 2400000.5) is VMS epoch  (MJD 0)
//   01-Jan-1958 (JD 2436204.5) is TAI epoch  (MJD 36204)
//   01-Jan-1900 (JD 2415020.5) is NTP epoch  (MJD 15020)
//Â Â  01-Jan-1970 (JD 2440587.5) is UNIX epoch (MJD 40587)
//   06-Jan-1980 (JD 2444244.5) is GPS epoch  (MJD 44244)
//   01-Jan-2000 (JD 2451544.5) is Y2K epoch  (MJD 51544)

The leading decimal digit of MJD changes only once every ~27 years. MJD 
40000 happened during 1968, MJD 50000 happened during 1995, and we all 
get to experience MJD 60000 happening right now in 2023. It's not a 
dangerous "rollover" or "overflow" event, but simply a rare chance to 
see a change in leading digit. Like a decade birthday; something to 
celebrate.

Many of us are used to seeing a 5 digit number starting with 5 and know 
it's a date. You'll now get used to seeing MJD numbers in the 60k range. 
Note that the entire history of time-nuts, and even the Tom Clark 
mailing lists before it, occurred during the 50k era of MJD.

Again, I don't expect any serious problems, but keep your eyes open for 
anything strange 5 days from now at:

2023-02-25 00:00:00 UTC (DOY = 56, Sat, STOD = 0) = JD 2460000.50000 = 
MJD 60000.000000
Fri Feb 24 16:00:00 2023 (UTC-8 Pacific Standard Time)

p.s. Special prize to anyone who posts a photo of the MJD 60k event at 
home. The last time we had this much fun with MJD was [2]:

http://leapsecond.com/pages/mjd52325/

/tvb

[1] https://en.wikipedia.org/wiki/Julian_day

Note the historical origin of MJD and its relationship to both Sputnik 
and 36-bit mainframe computers.

[2] https://www.febo.com/pipermail/time-nuts/2006-April/020455.html