How did Romans write dates?

The Kalends were the first of the month.

20th February: Ante diem decimum (10 days before, counting inclusively) Kalendas Martias.

Only scholars used the count from the foundation of Rome. For most purposes, people would have used the consulship - so:

Georgio Busho et Ricardo Cheneio Consulibus VII

Maegan is correct, and you should award her best answer. The Romans set days of the month according to the nones, which was the 9th day, the ides, which was about the 15th day, and the kalends, which was the 23rd day. Other dates were referred to as so many days before or after those three dates. Romans referred to the year sometimes as so many years since the founding of the city, which they took to be our 753 bce. Other times, they used the names of the two consuls who served in that year. There was a joke when Julius Caesar was consul with a man named Bibulus that he was so active and Bibulus so less active that it was the consulship of Julius and Caesar.

It would be in Latin. Today would have been ante diem X Kalendas Martias. They didn't use numbers. Well, sort of.

with roman numerals