T.H.E.Y. have been pushing for metric (base 10) time for decades.
Metropolis was supposed to be a "smarter future", so of course they would use metric time.
But, humans really like the 12 hour days (and 12 hour nights) far better.
The 13 month calendar is intersting.
The year is 13 * 28 + 1 + ¼ - 1/13…
Meaning, every year starts on a different day of the week
Sun day
Moon day
and then each of the elements although i do not remember them well enough to not get them wrong.
Another 13 month calendar is based on moon cycles. Basically, it would start on New Moon (or maybe full moon)
The lunar calendar would help us regulate ourselves. They cycles in our life become more apparent.