On Tue, Apr 28, 2020 at 07:53:05PM -0500, Chris Muller wrote:
> I have a bunch of two-element Array log entries, where the first element is
> the timestamp, the second is the object being logged.  But these entries
> may never be consumed, so I wish to minimize their cost.  So, instead of
> storing the timestamps as DateAndTimes, I wanted to keep them as
> SmallIntegers.  I thought I could use Time utcMicroseconds, now a
> SmallInteger in 64-bit Squeak.
> 
> However, when I went to instantiate the DateAndTime lazily from that number
> it's said it's year 2089...
> 
>         DateAndTime utcMicroseconds: Time utcMicrosecondClock offset: 0
> " 2089-04-29T00:49:26.3326+00:00"
> 
> I realized this is because a different epoch being used (1901 vs. 1971)
> between the two.  This seems inconsistent and confusing, but that's less
> important to me at the moment than the efficiency I'm looking for.  Is
> there another way?
> 
>  - Chris

Store the time in units of milliseconds or microseconds relative to the
Posix epoch. This is an integer value (small integer in 64 bit Spur), and
it a common time basis in many operating system and database libraries,
so it is well understood and well defined.

At higher levels of precision (e.g. nanosecond), the value is not an
integer, but that is perfectly all right, just think of it as a Number
that is usually an integer in common use cases. If  you need to store
it in the database as an integer, just round it off to the nearest
millisecond or microsecond (whichever you have chosen).

Microsecond precision is probably a good choice. This would be measured
as microseconds elapsed since the Posix epoch. That gives you small
integer values on Spur 64 for reasonable time values, and it is a higher
level of precision than you will probably ever need.

Assuming that you want to use a measurement scale of seconds relative to
Posix epoch, and assuming also that you want to measure this in units
of microseconds, then:

	| now uSecRelativeToPosixEpoch newInstance |
	now := DateAndTime now.
	uSecRelativeToPosixEpoch := now utcMicroseconds.
	newInstance := DateAndTime utcMicroseconds: uSecRelativeToPosixEpoch offset: 0.
	now = newInstance "==> true"


Dave