# Gregorian Calendar
## Overview
- Civil calendar
- Epoch: 0001-01-01
- Variants: Proleptic, Cutover
- Day begins midnight
## Leap Year Rules
- Divisible by 4 → leap year
- Divisible by 100 → not leap year unless divisible by 400
- Examples:
- 2000 → leap
- 1800 → not leap
- 2004 → leap
## Conversion
- RD epoch arithmetic
- Y-M-D ↔ EpochDay
- Examples:
## Refs
- [Britannica](https://www.britannica.com/topic/Gregorian-calendar)