SOURCE_DATE_EPOCH and timezone with FAT images

[Thomas Schmitt]

Hi,

on occasion of

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1005197
  "pcmemtest: please make the build partly reproducible"

i wonder why
  https://reproducible-builds.org/specs/source-date-epoch/
does not mention "timezone" except by the somewhat riddling statement

  "Formatting MUST be deferred until runtime if an end user should
   observe the value in their own locale or timezone."

>From this i hope to correctly  argue in favor of a standard timezone for
software that is delivered as reproducible FAT filesystem image:

* Producers of FAT cannot avoid formatting of SOURCE_DATE_EPOCH to
  Year/Month/Day/Hour/Minute/Second fields.
* One cannot do this at runtime of the filesystem, which in the
  special case of pcmtest is when EFI reads the FAT filesystem to find and
  run the \EFI\BOOT\BOOT*.EFI start program.
=> So one must not let the end user see their own locale or timezone.

If this is correct, then i think there should be an explicit timezone rule
for reproducible images in formats like FAT, where it seems to be custom
to record timestamps in the local timezone.

When trying to convince programmers of the additional complexity it does
not really help that
  https://reproducible-builds.org/docs/source-date-epoch/
says:
  "At present, we do not have a proposal that includes anything
   resembling a "time zone"."
Although i agree that time representations are tricky, i think it is
necessary to specify a rule for the special case where the timezone
unavoidably influences the storage bytes of the reproducible software.


Have a nice day :)

Thomas