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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: GNAT Modification_Time limitation
Date: Tue, 20 Nov 2018 17:32:21 -0600
Organization: JSA Research & Innovation
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"Keith Thompson" <kst-u@mib.org> wrote in message 
news:lnefbgr0rz.fsf@kst-u.example.com...
...
> If you're on a non-POSIX system, there might still be a
> system-specific way to get a more precise timestamp.  (NTFS also
> seems to store timestamps with high precision.)

NTFS has three timestamps (modification, creation, and last access). Only 
the modification has high precision; the others are only good to full 
seconds (or something like that).

FAT file systems (as you might encounter on a camera or USB stick) only have 
precision to 2 seconds. (Which is why we had to deal with this in the 
Janus/Ada build tools fairly early on.)

Also note that the system clock on Windows systems typically only changes 
every 0.01 sec (Dmitry says this can be changed, although I've never seen 
that done). That extends to the file systems and other OS timers as well. 
Most Ada vendors use a Ada.Calendar.Clock that blends the system clock with 
the high performance timer to get useful accuracy of of Ada.Calendar.Time. 
(A customer/collaborator, Tom Moran, originally wrote that code the the 
Janus/Ada implementation of Calendar to fix some timing problem that he had. 
He eventually submitted similar code to AdaCore who added it to their 
Calendar as well.)

Moral: Doing "Make" on a modern machine, especially if you want it to be 
portable, is a tricky job.

                                  Randy.