Two right answers, and you need to know which you want
Ask how many days there are between the 1st and the 8th and there are two honest answers. The gap is 7. Counting both ends gives 8. Neither is wrong; they answer different questions, and most calculators pick one silently and leave you to find out the hard way.
Use the gap for anything measuring distance in time: days until a deadline, days remaining, days an invoice is overdue, notice given. Use the inclusive count for anything you are paying for or occupying: days on site, days on hire, nights in a hotel, days of cover on a policy.
The difference is one day, which sounds trivial until it is a day of plant hire on a twelve-week programme, or the day an invoice tips into late payment interest.
Months are not thirty days
A month on this page means a calendar month. One month from 15 January is 15 February, whether that turns out to be 28 days or 31. That is what a contract means, what a notice period means, and what a warranty means.
Dividing the day count by 30 drifts by about five days a year and gets progressively wronger the further out you go. It is the reason a "six month" figure and a "180 day" figure never quite agree.
Weekdays and weekend days
The split here is a plain calendar one: Saturday and Sunday are weekend days, everything else is a weekday. It takes no account of bank holidays, which is deliberate. If you need business days with the right public holidays for your nation, that is a different tool and the answer differs between England, Scotland and Northern Ireland.
No time of day in here, so nothing for the clocks to break
There is no time of day in here, so there is nothing for a time zone or the clocks changing to break. Everything is calendar dates, handled in UTC, and a date is the same date wherever you are reading this.
It also refuses dates that do not exist. Type 30 February and it will say so rather than quietly treating it as 2 March and giving you a confident answer about the wrong day, which is what the underlying browser date handling does if you let it.
Common questions
How many days are there between two dates, say the 1st and the 8th?
There are two honest answers. The gap between them is 7 days. Counting both ends gives 8. Neither is wrong, they answer different questions, and most calculators pick one silently and leave you to find out which the hard way.
Should I use the gap or the inclusive count for something like plant hire or a hotel stay?
Use the inclusive count, which counts both ends, for anything you are paying for or occupying: days on site, days on hire, nights in a hotel, days of cover on a policy. Use the plain gap for anything measuring distance in time instead, such as days until a deadline or days an invoice is overdue.
Why is a month not simply 30 days?
Because a month on this page means a calendar month: one month from 15 January is 15 February, whether that turns out to be 28 days or 31, which is what a contract, a notice period or a warranty actually means. Dividing a day count by 30 instead drifts by about five days a year and gets progressively more wrong the further out you go.
Does the calculator account for bank holidays when counting weekdays?
No, deliberately. The weekday and weekend split here is a plain calendar one, Saturday and Sunday as weekend days and everything else a weekday, with no account taken of bank holidays, since business days with the right public holidays differ between England, Scotland and Northern Ireland and need a different tool.
What happens if I type in a date that does not exist, like 30 February?
It refuses the date and says so, rather than quietly treating it as 2 March and giving a confident answer about the wrong day, which is what the underlying browser date handling does if you let it.
Does this calculator deal with time zones or the clocks changing?
There is no time of day involved at all, so there is nothing for a time zone or a clock change to break. Every date is handled in UTC as a calendar date, so a date is the same date wherever you are reading this.