Date Difference Calculator

Calculating date differences — more complex than it looks

Counting days between two dates seems simple, but edge cases make it surprisingly tricky: leap years, different month lengths, time zones, and the question of whether you count both the start and end date or just one of them.

The Australian financial year adds another layer — many calculations in a business context use the financial year (1 July to 30 June) as the reference period rather than the calendar year.

• Leap years: A year is a leap year if divisible by 4, except centuries must be divisible by 400. 2000 was a leap year; 1900 was not. The next non-leap century year is 2100.
• Financial year: Australia's tax year runs 1 July – 30 June. Most business reporting, super contribution caps, and tax offsets reset on 1 July.
• Calendar reform: Australia adopted the Gregorian calendar in 1752 (with Britain). Before that, the Julian calendar was used, which runs 11 days behind — important for historical date calculations.
• Business days: Date difference in business days requires knowing state-specific public holidays, which vary significantly across Australia's states and territories.