Dates & Time
Counting days, ages and working hours — where the arithmetic is genuinely ambiguous, and how to say which convention you meant.
A date is a day count in disguise — count the days since 1 January 1970, multiply by 86,400, and you have the same number the tool prints.
No calendar app needed — subtracting serial day numbers (or adding up the days left in each month) gets you the same answer as a countdown timer.
Add 280 days to the first day of the last period and you have the standard estimate — here's the arithmetic, a worked example, and why only about 1 in 20 babies actually arrive on that exact day.
How many days from Monday to Friday depends entirely on whether you count both ends. Both answers are correct arithmetic, and the ambiguity is worth naming out loud.
An age in years depends on whether the birthday has already happened this year, and one month after 31 January has no single correct answer. Here is how the calculation actually proceeds.
Counting weekdays between two dates is exact arithmetic. Deciding which days are the weekend, and which holidays you observe, is not something any date can tell you.
The difference between two cities' clocks changes during the year, because daylight saving starts and ends on different dates in different places and runs the opposite way south of the equator.
It is one integer: seconds since 1970 in UTC. Every time-zone bug and every accidental trip to the year 57971 comes from what happens after that.