What a Month Means in a Deadline
Thirty days and one month are different periods, and the difference is up to three days depending on where in the year they fall. Most systems count months to the corresponding date, which works until the corresponding date does not exist.
The rule in short
A period expressed in months runs to the same day-number in the later month rather than by adding a fixed number of days. Where the later month is shorter and has no corresponding date, the period generally ends on its last day. Periods expressed in days are counted in days regardless of how many months they span, and mixing the two conventions is a common source of error.
A period of one month sounds like the easiest arithmetic in procedure and is not. Counting thirty days produces one date, counting to the same day-number produces another, and the gap widens around February.
The corresponding date rule
Same day-number, next month. One month from the fourteenth of a month ends on the fourteenth of the next, whatever the number of days between them.
Several months work the same way. Three months from the second of a month ends on the second, three months later, without any counting of days.
Years follow the same pattern. A period of years ends on the same date in the later year, which is dealt with in periods measured in years.
The exclusion of the first day still applies. The corresponding date rule fixes the end of the period; the day the event occurred is still excluded from it.
It is not thirty days. Thirty days from the first of January ends on the thirty-first; one month ends on the first of February, two days later.
The difference is largest around February. One month from the thirty-first of January and thirty days from the same date can end three days apart.
The unit is chosen deliberately. A drafter who writes months has picked a period that tracks the calendar, and one who writes days has picked one that does not.
Where the corresponding date does not exist
The thirty-first problem. One month from the thirty-first of a month leaves no corresponding date in a thirty-day month.
The usual answer is the month end. Most systems treat the period as ending on the last day of the shorter month rather than spilling into the next one.
February compresses it further. One month from the twenty-ninth, thirtieth or thirty-first of January ends at the end of February in most years.
Leap years shift it again. The twenty-ninth exists in some years and not others, which matters for periods of one year from that date.
Some instruments say otherwise. A rule or contract can specify the first day of the following month instead, and where it does that wording governs.
The convention is worth checking once. The forum's computation provision usually answers it, and the answer then holds for every period in that forum thereafter.
Write the resolved date down. Once the month-end question has been decided for a particular period, recording the answer stops it being rediscovered under pressure.
| Period from 31 January | Ends | Why |
|---|---|---|
| 30 days | 2 March | Fixed count of days |
| 1 month | 28 or 29 February | No corresponding date |
| 2 months | 31 March | Corresponding date exists |
| 90 days | 1 May | Fixed count of days |
| 3 months | 30 April | No 31st in April |
Periods in days against periods in months
Days ignore months entirely. A ninety-day period is ninety days wherever it starts, and it lands on different dates depending on the months it crosses.
Ninety days is not three months. The two periods differ by up to two days, which matters on any deadline that is being used to its limit.
Rules choose deliberately. A drafter who writes days rather than months has usually chosen a period that does not move with the calendar.
Mixed clauses cause disputes. An agreement giving thirty days for one step and one month for the next has two conventions running in the same document.
Convert once and record it. Working out the end date and diarizing it removes the need to remember which convention applied.
Do not treat them as interchangeable. The habit of saying thirty days when a rule actually says one month is how a wrong date reaches a diary and stays there.
Check which unit a letter uses. Correspondence granting somebody a period should say days or months explicitly, because the recipient will otherwise pick whichever is longer.
The two conventions produce dates up to three days apart, and the gap always runs in the direction of the shorter month. Any period expressed in months that starts near a month end deserves the calculation written out rather than done in passing.
Common situations
A monthly obligation. Where something is due each month, the corresponding date recurs, and the month-end problem recurs with it.
A period of one month for a response. Short administrative windows are frequently expressed in months, which makes the start date matter more than usual.
Periods that follow a closure. Where the corresponding date falls on a weekend or holiday, the adjustment applies as normal, per weekends, holidays and closures.
Renewals and anniversaries. Annual obligations run to the same date, and the leap-year date is the one that needs a decision in advance.
Notice periods before an event. A month's notice before a hearing is counted backwards, which inverts the arithmetic, per deadlines that run backwards.
Contracts drafted abroad. Other systems count months in the same way but resolve the month-end problem differently, which is worth confirming.
Getting it right
Read the unit the rule uses. Days, months and years are three different instructions, and the rule says which one applies.
Identify the trigger first. The corresponding date depends entirely on the start date, which is the exercise in when the clock actually starts.
Check the month end deliberately. Where the start date is the twenty-ninth or later, the end of the period needs a moment's thought rather than a glance.
Write the end date in full. A diary entry saying one month is a calculation waiting to be redone; one showing the date is a fact.
Note the leap year where it matters. For a period of one year from the twenty-ninth of February, the answer should be decided before it is needed.
Leave a margin at the month end. The whole class of problem disappears if the step is taken a few days before the corresponding date.
Periods expressed in months run to the corresponding date in the later month, not by adding a fixed number of days, and the two methods produce different answers.
The convention needs a rule only where the corresponding date does not exist, and the usual answer is the last day of the shorter month rather than the first day of the next one.
Periods expressed in days ignore the calendar entirely, which is usually why a drafter chose them, and treating ninety days as three months introduces an error of a day or two.
The situations that catch people are predictable: start dates near a month end, annual periods from the twenty-ninth of February, and agreements that use both conventions in adjacent clauses.
The practical protection is to convert the period into a written date as soon as the trigger is known, and to leave a margin where the arithmetic runs through the end of a short month.
Points to carry away
- A month runs to the same date in the following month.
- Thirty days and one month are not the same period.
- Where no corresponding date exists, the month end is used.
- Periods in days ignore month boundaries entirely.
- Contracts frequently mix the two conventions in one clause.
Questions readers ask
Is one month the same as thirty days?
No, and the difference can be three days. A period of one month is normally counted to the same day-number in the following month, so one month from the fifteenth ends on the fifteenth regardless of whether the month has twenty-eight or thirty-one days. Thirty days is a fixed count that ignores the calendar. The two conventions diverge most sharply around February, and a rule that says months has chosen a period that moves with the calendar rather than one that does not.
What happens if the next month has no corresponding date?
In most systems the period ends on the last day of that month. One month from the thirty-first of January therefore ends on the twenty-eighth or twenty-ninth of February, and three months from the thirty-first of January ends on the thirtieth of April. Some instruments specify the first day of the following month instead, so where the start date is the twenty-ninth or later it is worth checking the computation provision rather than assuming the ordinary answer.
Does the exclusion of the first day still apply to a month?
Yes. The corresponding date rule fixes where the period ends; it does not change the convention about where it begins. The day the triggering event occurred is still excluded, and the period runs from the following day to the corresponding date in the later month. Where that corresponding date falls on a weekend or a holiday, the ordinary closure adjustment applies and moves the deadline to the next day the office is open.
Sources
- Federal Rules of Civil Procedure — Rule 6, Computing and Extending Timelaw.cornell.edu
- 1 U.S.C. 1 — Rules of Constructionlaw.cornell.edu
- Federal Rules of Appellate Procedure — Rule 26, Computing and Extending Timelaw.cornell.edu
- Federal Rules of Bankruptcy Procedure — Rule 9006, Timelaw.cornell.edu
- Legal Information Institute — Computation of Timelaw.cornell.edu
- Federal Rules of Criminal Procedure — Rule 45, Computing and Extending Timelaw.cornell.edu
Urban Justice Docket is a publication, not a law firm. This article states general rules and cites its sources; it is not advice about any particular case, and the law differs by state and changes over time.
More in Counting the Days
When the Clock Actually Starts Running
Every time limit has a trigger, and the trigger is defined by the rule rather than by common sense. Service, issue, receipt, demand and completion are all different events, and a period counted from the wrong one is wrong by however many days separate them. Finding the trigger in the text is the first step in any deadline question.
Weekends, Holidays and Closures
Where the last day of a computed period falls on a weekend, a holiday or a day the filing office is inaccessible, the period generally extends to the next day it is open. The extension applies to the end of a period rather than to days inside it, and an office being busy, understaffed or closed to visitors is not the same as being inaccessible.
Deadlines That Run Backwards From an Event
A period counted back from a future event is computed in the opposite direction: the day of the event is excluded, the days are counted backwards, and a last day falling on a closure moves earlier rather than later. Additional time for service also moves the deadline earlier. The arithmetic is simple and the instinct to apply the forward conventions is strong.