How many days has it been? Counting from a date, and why the counter works
Three months is 89 days once and 92 another time. The starting day usually does not count. And a counter can turn an intention into something you would rather not break - as long as it is not the only thing it shows.
"I have been on a diet since the first of September." A simple sentence with three questions behind it: how many days is that, when does it end, and what is the counter actually measuring.
The starting day counts as zero
This is not only a calculator convention. Polish law counts the same way: for a deadline set in days, the day on which the event happened is not counted (art. 111 of the Civil Code). A contract signed on Monday with a seven day deadline ends the following Monday, not on Sunday.
In practice: something that began yesterday has lasted one day. On the starting day the counter shows zero, which is correct even though we would say "day one" out loud.
Three months is not one number
A calendar month runs from 28 to 31 days, so "three months" means different things depending on when you start:
| start | end | days |
|---|---|---|
| 1 January | 1 April | 90 |
| 1 February | 1 May | 89 |
| 1 June | 1 September | 92 |
| 1 July | 1 October | 92 |
Three days of spread. That is why the counter takes a month as 30 days and three months as 90: then "three months" and "90 days" mean the same thing and there is no argument about whether the plan is done.
Official deadlines work the other way round: there the calendar decides, and a deadline set in months ends on the day with the same number in the final month.
Why count at all
A counter changes what an intention is. Instead of "I want to lose weight" there is a number that grows by itself and that you would rather not reset - the same mechanism that keeps people from breaking a streak in a language app.
It is worth knowing the other side of it. In the 2010 study by Lally and colleagues, missing a single day had no material effect on habit formation; repetition over time was what mattered. A counter that shows only an unbroken run therefore teaches something the data does not support: that one slip sends you back to the beginning. Which is why resetting our counter keeps the record, and why a plan shows a percentage rather than a bare number of days.
More on how long habit formation actually takes - a median of 66 days, not 21
- is in a separate piece: how long to form a habit.
What is worth counting
- Things already under way. No need to wait for Monday: put in a date in the past and the counter starts with what you have already done.
- Things with no end. "Not smoking since" needs no plan to make sense.
- Things with a deadline. A 30 day challenge, three months without alcohol, a hundred days of study: there the percentage says more than the raw number.
What not to count: anything whose progress is measured in money or in repetitions. Those are goals, not counters - a goal takes a value, a counter takes only a date.
Count your own days
The day counter works without an account: enter a date, optionally a plan, and see the days behind you and the percentage of the plan. If you are counting forward - to the holidays, to Christmas, to an exam - that is what the countdown is for.