How it is worked out
weeks = days between the dates ÷ 7
Why it is done this way
Weeks are the one calendar unit with a fixed length, which makes them the easiest to calculate and the most reliable to plan with. Divide the days by seven: the whole part is the weeks and the remainder is the days left over.
Step by step
- Count the calendar days between the two dates
- Divide by seven
- The whole part is the number of complete weeks
- The remainder is the days left over
What is worth knowing
Because a week is exactly seven days, this is the only date calculation with no edge cases at all — no month lengths, no leap years, nothing. That reliability is why gestational age is counted in weeks and days rather than months, and why project plans are built on them. The working-week figure divides by five instead of seven, which is the useful number when the question is how much time there is to get something done rather than how much time will pass.