For example,
60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day
7 day = 1 week
29-31 days = Month (approx.)
365/366 days = year
It's like for the imperial measurement of distance, where 1 mile = 5280 feet...
Edit: just to clarify, I'm more or less keen towards any consistent, decimal-based measurement systems like base-10 or base-12.
It's actually January and February that were added in. Before that, there was just this gap during the winter when they didn't bother counting the passage of days until the start of spring was declared, in March. That's also why February is the short month, as it was the last added and just used up the remaining days.
Worth emphasizing that the year began in March – December means the 10th month because it was the 10th month, even after January/February were added
There were also superstitions about February - due to its role as a time for ritual purification and cleansing, that made them reticent to add days to it, so it stayed at the bare 28