Using a deterministic value breaks Windows' AppCompat database in strange ways.
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The TL;DR is that by default we need to write a real timestamp. We can still get build determinism without breaking AppCompat, but the application will need to specify an explicit value to use for the time stamp. So the method of always using a hash of the binary by default is changed to using the time stamp by default, but we add an option to use an explicit value or the old method of using a hash.