Experimental Physics and
Industrial Control System

"Hill, Jeffrey O" <[email protected]> · Wed, 15 Feb 2012 16:54:17 +0000

Hi Frank,

The EPICS epoch is 1/1/1990 00:00:00UTC, and therefore one can compute the time of the 32 bit _unsigned_ integer seconds rollover event as follows. 

0xffffffff sec / (60 sec/min * 60 min/hour * 24 hour/day * 360 day/year)

By my calculations (check them for yourself as I just now ran the numbers as I was writing this mail) this is about 138 years after the epoch which is about 2128.

The codes (see src/libCom/osi/epicsTime.{cpp,h}) that computes time stamp differences, and compares timestamps, are carefully written so that they produce robust results when the operands are on either side of a time stamp rollover as long as the difference between the operands does not exceed one half of the full range.

Therefore we hope that projects can operate in time frames near the rollover event as long as they don't subtract time stamps that have more than 69 years between them.

Jeff

> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> On Behalf Of Di Maio Franck
> Sent: Wednesday, February 15, 2012 8:27 AM
> To: [email protected]
> Subject: Precise time beyond 2038
> 
> Dear all
> 
> Just to share a worry, get corrections if I am wrong & ideas if any.
> 
> ITER will run 20+ years or more, so we shall avoid the 2038 limit
> encountered by time representation that use SIGNED 32 bits integer for
> seconds (cf. http://en.wikipedia.org/wiki/Unix_time).
> 
> My understanding is that while time stamps are signed 32 bits integer (+ a
> second integer for nanoseconds fraction) it should be OK ....up to 2106
> (1970+136y).
> 
> But to program a "future time event" via CA beyond 2038, no way to use an
> integer for seconds (cannot have better than a signed 32 bits integer). So
> one has to use at least one double.
> 
> Many variants are possible. With a mantissa of 52 bits (IEEE-64 floats),
> one could in theory use seconds fractions up to 20 bits (microseconds) but
> I expect it is not as simple as that. Anyway, we also need nanoseconds so
> we'll split the time in 2 parts (ex: base + delay).
> 
> The device support use 64 bits for time in nanos and our OS is Linux
> x86_64 bits so no other limits.
> 
> I'll appreciate comments from EPICS freaks having face similar questions.
> 
> Cheers,
> Franck
> 
> Franck Di Maio
> CODAC System Designer
> CODAC Section
> 
> ITER Organization, Building 519/027, CHD, Control System Division
> Route de Vinon sur Verdon - 13115 St Paul Lez Durance - France
> Phone: +33 4 42 17 64 05

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System