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Replace int64_t usages with 'int' instead. (#359)

Previously the constants used for converting between
different units of time were declared using int64_t. However
we should only use explicitly sized integer types when they
are required, and should use 'int' everwhere else, and there is
no good reason to use int64_t here.

For that reason this patch changes the type of the constants.
This should help address issue #354 as well.