[HECnet] Strange unit of time

[Mark Matlock]

Ian,
   You are thinking about “clunks”. Below is a nice article from a VMS wiki.
Best,
Mark

Timekeeping	Edit
OpenVMS represents system time as the 64-bit number of 100 nanosecond intervals (that is, ten million units per second; also known as a 'clunk'[52][53]) since the epoch. The epoch of OpenVMS is midnight preceding November 17, 1858, which is the start of Modified Julian Day numbering. The clock is not necessarily updated every 100 ns; for example, systems with a 100 Hz interval timer simply add 100000 to the value every hundredth of a second. The operating system includes a mechanism to adjust for hardware timekeeping drift; when calibrated against a known time standard, it easily achieves an accuracy better than 0.01%. All OpenVMS hardware platforms derive timekeeping from an internal clock not associated with the AC supply power frequency.

While the system is shut down, time is kept by a Time-of-Year ("TOY") hardware clock. This clock keeps time to a lower resolution (perhaps 1 second) and generally, a lower accuracy (often 0.025% versus 0.01%). When the system is restarted, the VMS 64-bit time value is recomputed based on the time kept by the TOY clock and the last recorded year (stored on the system disk).

The 100 nanosecond granularity implemented within OpenVMS and the 63-bit absolute time representation (the sign bit indicates absolute time when clear and relative time when set) should allow OpenVMS trouble-free time computations up to 31-JUL-31086 02:48:05.47. At this instant, all clocks and time-keeping operations in OpenVMS will suddenly fail, since the counter will overflow and start from zero again.

Though the native OpenVMS time format can range far into the future, applications based on the C runtime library will likely encounter timekeeping problems beyond January 19, 2038 due to the Year 2038 problem. Many components and applications may also encounter field-length-related date problems at year 10000 (see the Year 10,000 problem).[54]



Mark

> On Apr 23, 2018, at 5:35 PM, Ian McLaughlin <ian at platinum.net> wrote:
> 
> Excuse the noise, but I’ve got a fragmented recollection of a strange unit of time, and my faulty memory seems to equate this with VAX and/or VMS. My google-fu has failed me (or confirmed that this is some sort of fake memory) however I thought I’d run it past the experts in here.
> 
> Does anyone know of anything strange about the unit of time, or possibly the epoch, or something else associated with time measurement (or the TOY clock or basically anything to do with time) in the DEC world? Sorry for the vagueness of the question, but hopefully someone can help :)
> 
> Ian
> 
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