Why this matters
One month notice can end on a different date from 4 weeks notice. Use the unit in your contract.
A calendar month is not 30 days
One month's notice ends on the corresponding date in the following month, not 30 days later. Notice given to take effect on 15 October ends on 14 November (if the day of service is not counted) or 15 November (if it is).
The difference matters in February and in any month with 30 days: a 30-day calculation and a calendar-month calculation diverge by a day or two.
Worked example
Notice given on Wednesday 14 October 2026, effective the next day, ends on Saturday 14 November 2026. If the contract rolls the end date to a working day, it becomes Monday 16 November.
Set the start date to the date notice takes effect, not the date you wrote it, and the calculator will apply the calendar-month rule from there.
Check the contract for the counting rule
Contracts differ on whether the notice day counts, whether part-months round up, and whether the end date must be a working day. The arithmetic here is the calendar-month span; the contractual rule decides which dates it runs between.
Months are not the same length
A calendar month varies between 28 and 31 days, so the same notice given in different months produces different end dates. Notice effective on 31 January ends on 28 February in a common year and 29 February in a leap year. Notice effective on 30 April ends on 30 May. A 30-day calculation would miss both.
This is why one month and 30 days are not interchangeable, and why employers who calculate in days sometimes end a notice period a day early or late.
Part months and rounding
Where notice starts part-way through a month, the end date is the corresponding date in the following month rather than the end of that month. Some contracts add a rule that a part month counts as a whole month, which pushes the end date later still.
If a contract is silent, the ordinary calendar-month meaning applies. If it contains a rounding clause, use the start date field to set the effective date and check the clause against the result.
When the end date is not a working day
Many contracts provide that notice ending on a weekend or bank holiday rolls to the next working day. Others do not, and the period simply ends on that date. The difference can be two or three days over a Christmas period, so it is worth knowing which applies before you plan around the date.
FAQs
Is 1 month always 30 days?
No, it depends on the calendar month.
Should I use 4 weeks instead?
Only if your contract says four weeks, not one month.