**More divisors, as mentioned above.**
Thinking along that line -- what would a half year, a quarterly tax file date, bimonthly, etc. etc.
Who cares? If you go Metric, the phrase "A miss is as good as a mile" loses its meaning. Some things would have to be adapted.
Easy conversions among most calendars is to find the Julian date. Some desk calendars have them written as days since year began, or days remaining in year.
More complicated is converting to a decimal form, but that would be much like converting from Celcius to Fahrenheit (and for God's sake, don't get me started on Fahrenheit!!!)
We'll need to do conversions long before we travel between the stars. NASA is already dealing with the problem as it communicates and navigates the robots on Mars. The fourth planet has a different day length, different seasons, et cetera.
When we have settlements on Mars, and someone has a birthday, break out the calculator first!!