I don't see any reasons why the arguments of os.epoch should be case-sensitive.
This could be a problem especially with UTC when people try os.epoch("UTC"), but I think it would also make sense for "local" and "ingame" to be checked case-insensitive.
It might be worth doing .toLowerCase(Locale.ROOT) instead (or Locale.ENGLISH) so you don't get strange behaviour on non-English systems. This change should probably be made to os.time and os.day as well (oh goodness, who thought the time changes were a good idea?).
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