Improved leap year support

Leap year condition:

• if year is not divisible by 4:
  • then: it is a common year
• else if year is not divisible by 100:
  • then: it is a leap year
• else if year is not divisible by 400:
  • then: it is a common year
• else:
  • it is a leap year

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Currently implemented: If a year is divisible by 4, it is a leap year; otherwise it is a common year.