Date Difference Calculator

Date Calculation Guide

Days vs Working Days

Calendar days count every day including weekends and bank holidays. Working days (also called business days) count only Monday to Friday. A 30-day period typically contains 22 working days. For contract deadlines, payment terms, and regulatory timeframes, check whether 'days' means calendar or working days — in law and finance, it often means calendar days unless otherwise specified, but employment-related deadlines (P45, notice periods) typically use calendar days.

Counting Months and Years

Month-based calculation is more complex than day-based because months have different lengths. Going from January 31 to February 28 is one month, but 28 days. Going from January 1 to February 1 is also one month but 31 days. The ISO 8601 standard defines date arithmetic: months are counted by calendar position, not day count. For financial calculations (compound interest, loan amortisation), use exact day counts rather than month approximations.

Common Date Calculations

Notice periods (employment): typically 1 week per year of service (minimum), up to the contractual notice period. Start counting from the day after the notice is given. Project deadlines: if a project is '90 days', check whether this means calendar or working days — 90 working days is approximately 18 weeks vs 90 calendar days = ~13 weeks. Visa and passport validity: many countries require 6 months' validity beyond your travel dates — count carefully.

Age and Anniversary Calculations

Age calculations count complete years from birthday to the current date (or any target date). The leap year effect: someone born on February 29 officially celebrates their birthday on March 1 in non-leap years in most jurisdictions. Calculating a 25th anniversary from a given date involves adding exactly 25 years — if the original date was a leap day, the anniversary falls on March 1 or February 28 depending on jurisdiction and preference.