Unlike time in the real world, time in Europa Universalis II is quantized. The quantum of EU2-time is the day.
All months in EU2 have 30 days in them. Otherwise they are named the same and have the same order as real-world months. For all months except February, the 30 days are displayed as days 1 through 30 (what you'd expect). There are two days called "February 1", and another two called "February 15". These are complete, distinct days; they just happen to have the same label. You can pause on the "second day" of February 1 or February 15, movement is computed and stuff can happen, etc.
A year in EU2 is 12 months. Years are labelled by numbers, starting in 1419 and going through 1819.