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From: Florian Weimer <fw@deneb.enyo.de>
Newsgroups: comp.lang.ada
Subject: Re: How to cache output of the compiler aka ccache
Date: Fri, 18 Mar 2005 23:10:24 +0100
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* Randy Brukardt:

> Janus/Ada doesn't have the problem with the core runtime (since the source
> isn't supplied), but sometimes a daylight saving time change will force a
> rebuild of an entire project. I've never figured out a good solution, since
> the behavior is so odd.

If I read the specification of GetFileTime correctly, this
API does indeed return values relative to UTC.  Is this not the case?

Ah, found it:

| File Times and Daylight Saving Time
| 
| You must take care using file times if the user has set the system to
| automatically adjust for daylight saving time.
| 
| To convert a file time to local time, use the FileTimeToLocalFileTime
| function. However, FileTimeToLocalFileTime uses the current settings
| for the time zone and daylight saving time. Therefore, if it is
| daylight saving time, it will take daylight saving time into account,
| even if the file time you are converting is in standard time.
| 
| FAT records times on disk in local time. GetFileTime retrieves cached
| UTC times from FAT. When it becomes daylight saving time, the time
| retrieved by GetFileTime will be off an hour, because the cache has
| not been updated. When you restart the machine, the cached time
| retrieved by GetFileTime will be correct. FindFirstFile retrieves the
| local time from FAT and converts it to UTC using the current settings
| for the time zone and daylight saving time. Therefore, if it is
| daylight saving time, FindFirstFile will take daylight saving time
| into account, even if the file time you are converting is in standard
| time.
| 
| NTFS records times on disk in UTC. [...]

<http://msdn.microsoft.com/library/en-us/sysinfo/base/file_times.asp>

If there isn't any interface to read the file time as stored on FAT,
you lose.  Apparently, there isn't one.  Great engineering.