Jewish days were reckoned from sunset to sunset. The night time hours were divided into three "watches" of four hours each and the daylight hours were reckoned by twelve "hours". The Romans had a natural day of 12 day-hours and 12 night-hours. The first daylight hour (hora prima) began at sunrise, noon was the sixth hour (hora sexta), and the last hour (hora duodecima) ended at sunset. However the Romans also had a civil day which started at midnight. The day was divided into two parts: the ante meridiem and the post meridiem, before noon and afternoon, respectively.This is the system of am and pm which we still use now. Their night time hours were divided into four watches of three hours each.
| Jewish Hour | Roman Natural | Roman Civil | 24-Hour |
| Sunrise | Sunrise | 6th hour AM | 06:00 |
| First hour | First hour | 7th hour AM | 07:00 |
| Second hour | Second hour | 8th hour AM | 08:00 |
| Third hour | Third hour | 9th hour AM | 09:00 |
| Fourth hour | Fourth hour | 10th hour AM | 10:00 |
| Fifth hour | Fifth hour | 11th hour AM | 11:00 |
| Sixth hour | Sixth hour | 12th hour | 12:00 |
| Seventh hour | Seventh hour | 1st hour PM | 13:00 |
| Eighth hour | Eighth hour | 2nd hour PM | 14:00 |
| Ninth hour | Ninth hour | 3rd hour PM | 15:00 |
| Tenth hour | Tenth hour | 4th hour PM | 16:00 |
| Eleventh hour | Eleventh hour | 5th hour PM | 17:00 |
| Twelth hour | Twelth hour | 6th hour PM | 18:00 |
| First watch (18:00-22:00) | First watch (18:00-21:00) | ||
| Second watch (22:00-2:00) | Second watch (21:00-24:00) | ||
| Third watch (2:00-6:00) | Third watch (24:00-3:00) | ||
| Fourth watch (3:00-6:00) |