[9.20] fix: dev: Change the NS_PER_SEC (and friends) from enum to static const
New version of clang (19) has introduced a stricter checks when mixing integer (and float types) with enums. In this case, we used enum {} as C17 doesn't have constexpr yet. Change the time conversion constants to be static const unsigned int instead of enum values.
Closes #4845 (closed)
Backport of MR !9313 (merged)