How do I add months to a date?

Move the month forward and keep the same day number. If that day does not exist in the new month, the date clamps back to the last day of it.

What is one month after 31 January?

28 February, or 29 February in a leap year. There is no 31 February, and the convention everywhere is to clamp rather than roll into March.

What is 6 months from today?

Set the unit to months and the amount to 6. It works out at about 182 days, though the exact span depends on which months are crossed.

What is the difference between adding 30 days and adding a month?

They rarely match. Thirty days from 31 January is 2 March, but one month from 31 January is 28 February. Check which one your contract means.

How do I subtract years from a date?

Set the unit to years and the direction to subtract. Leap days are handled, so 29 February minus one year gives 28 February.

How many weeks are in a month?

About 4.35. A month is not four weeks, which is why a four-week cycle drifts away from monthly dates over a year.

What is one year from 29 February?

28 February in the following year, because there is no 29 February to land on except every fourth year.

Should a notice period be counted in days or months?

Read the wording. A three-month notice and a 90-day notice can land on different dates, and the difference has been argued in court.

Does this count weekends and holidays?

Every day is counted here. For a deadline that skips weekends, use the date calculator with the working days option.

Does this calculator store anything?

No. It runs entirely in your browser and nothing you type leaves the page.

Calculator

Date Add and Subtract Calculator

Add or take off years, months, weeks or days from any date. Months are handled the way contracts expect: 31 January plus one month lands at the end of February, never in March.

Start dateDefaults to todayAmountHow many to add or take off

UnitWhat to count inDirectionForwards or backwards

—

Pick a date and an amount

Day of the week

—

Where it lands

Days apart

Actual calendar days

Counted

—

Added

Reference

Common spans in weeks and months

Useful when a contract gives a span in days but you think in months. Each row converts days into weeks, months, and roughly how many working days.

1 to 30 days1 to 30 days written as weeks, months and roughly how many working days.

1 to 30 days written as weeks, months and roughly how many working days.
Span	Weeks	Months	Working days
1 days	0.14	0.03	about 1
2 days	0.29	0.07	about 1
3 days	0.43	0.1	about 2
4 days	0.57	0.13	about 3
5 days	0.71	0.16	about 4
6 days	0.86	0.2	about 4
7 days	1	0.23	about 5
8 days	1.14	0.26	about 6
9 days	1.29	0.3	about 6
10 days	1.43	0.33	about 7
11 days	1.57	0.36	about 8
12 days	1.71	0.39	about 9
13 days	1.86	0.43	about 9
14 days	2	0.46	about 10
15 days	2.14	0.49	about 11
16 days	2.29	0.53	about 11
17 days	2.43	0.56	about 12
18 days	2.57	0.59	about 13
19 days	2.71	0.62	about 14
20 days	2.86	0.66	about 14
21 days	3	0.69	about 15
22 days	3.14	0.72	about 16
23 days	3.29	0.76	about 16
24 days	3.43	0.79	about 17
25 days	3.57	0.82	about 18
26 days	3.71	0.85	about 19
27 days	3.86	0.89	about 19
28 days	4	0.92	about 20
29 days	4.14	0.95	about 21
30 days	4.29	0.99	about 21

31 to 120 days31 to 120 days written as weeks, months and roughly how many working days.

31 to 120 days written as weeks, months and roughly how many working days.
Span	Weeks	Months	Working days
31 days	4.43	1.02	about 22
32 days	4.57	1.05	about 23
33 days	4.71	1.08	about 24
34 days	4.86	1.12	about 24
35 days	5	1.15	about 25
36 days	5.14	1.18	about 26
37 days	5.29	1.22	about 26
38 days	5.43	1.25	about 27
39 days	5.57	1.28	about 28
40 days	5.71	1.31	about 29
41 days	5.86	1.35	about 29
42 days	6	1.38	about 30
43 days	6.14	1.41	about 31
44 days	6.29	1.45	about 31
45 days	6.43	1.48	about 32
46 days	6.57	1.51	about 33
47 days	6.71	1.54	about 34
48 days	6.86	1.58	about 34
49 days	7	1.61	about 35
50 days	7.14	1.64	about 36
51 days	7.29	1.68	about 36
52 days	7.43	1.71	about 37
53 days	7.57	1.74	about 38
54 days	7.71	1.77	about 39
55 days	7.86	1.81	about 39
56 days	8	1.84	about 40
57 days	8.14	1.87	about 41
58 days	8.29	1.91	about 41
59 days	8.43	1.94	about 42
60 days	8.57	1.97	about 43
65 days	9.29	2.14	about 46
70 days	10	2.3	about 50
75 days	10.71	2.46	about 54
80 days	11.43	2.63	about 57
85 days	12.14	2.79	about 61
90 days	12.86	2.96	about 64
100 days	14.29	3.29	about 71
110 days	15.71	3.61	about 79
120 days	17.14	3.94	about 86

121 to 365 days121 to 365 days written as weeks, months and roughly how many working days.

121 to 365 days written as weeks, months and roughly how many working days.
Span	Weeks	Months	Working days
130 days	18.57	4.27	about 93
140 days	20	4.6	about 100
150 days	21.43	4.93	about 107
160 days	22.86	5.26	about 114
180 days	25.71	5.91	about 129
200 days	28.57	6.57	about 143
210 days	30	6.9	about 150
240 days	34.29	7.89	about 171
270 days	38.57	8.87	about 193
300 days	42.86	9.86	about 214
330 days	47.14	10.84	about 236
360 days	51.43	11.83	about 257
365 days	52.14	11.99	about 261

More than a yearMore than a year written as weeks, months and roughly how many working days.

More than a year written as weeks, months and roughly how many working days.
Span	Weeks	Months	Working days
400 days	57.14	13.14	about 286
456 days	65.14	14.98	about 326
500 days	71.43	16.43	about 357
547 days	78.14	17.97	about 391
600 days	85.71	19.71	about 429
730 days	104.29	23.98	about 521
900 days	128.57	29.57	about 643
1,000 days	142.86	32.85	about 714
1,095 days	156.43	35.98	about 782
1,460 days	208.57	47.97	about 1,043
1,825 days	260.71	59.96	about 1,304
3,650 days	521.43	119.92	about 2,607

Adding a month is a convention, not a sum

There is no 31 February, so one month after 31 January has no arithmetic answer. Someone has to decide, and the decision everyone has settled on is to clamp to the last day of the target month: 28 February, or 29 in a leap year.

That is what this calculator does, and it is worth knowing because naive code does the opposite. Ask most date libraries for 31 January plus a month and they spill into 2 or 3 March, which is not what any contract means.

The same clamping applies to years. One year after 29 February is 28 February, because the day it is trying to land on only exists every fourth year.

Months and days are not interchangeable

A three-month notice period and a 90-day notice period are different lengths, and which one you have depends entirely on the wording. From 1 January, three months lands on 1 April, while 90 days lands on 1 April too in a leap year and 31 March otherwise.

The gap widens over longer spans. Six months from 1 January is 1 July, which is 181 days, but 180 days from 1 January is 30 June. One day, but it is the day a renewal either auto-extends or does not.

A month is also not four weeks. It averages 4.35, which is why anything billed on a four-week cycle drifts away from calendar months and ends up with thirteen billing periods in a year instead of twelve.

The dates that matter are usually about money

The reason people add six months to a date is rarely curiosity. It is a renewal, a notice period, a review, or a contract end, and each of those arrives with a question about what happened in the meantime.

Arriving at that date with the numbers already in hand is the difference between a renewal conversation and a scramble. Reports keep the hours per client and project up to date as the work happens.

For plain day counting, the date calculator adds and subtracts days including working days only, and the date difference calculator measures the gap between two dates you already have.

Answers

Frequently asked questions

Common queries and answers about the date add and subtract calculator.

How do I add months to a date?: Move the month forward and keep the same day number. If that day does not exist in the new month, the date clamps back to the last day of it.
What is one month after 31 January?: 28 February, or 29 February in a leap year. There is no 31 February, and the convention everywhere is to clamp rather than roll into March.
What is 6 months from today?: Set the unit to months and the amount to 6. It works out at about 182 days, though the exact span depends on which months are crossed.
What is the difference between adding 30 days and adding a month?: They rarely match. Thirty days from 31 January is 2 March, but one month from 31 January is 28 February. Check which one your contract means.
How do I subtract years from a date?: Set the unit to years and the direction to subtract. Leap days are handled, so 29 February minus one year gives 28 February.
How many weeks are in a month?: About 4.35. A month is not four weeks, which is why a four-week cycle drifts away from monthly dates over a year.
What is one year from 29 February?: 28 February in the following year, because there is no 29 February to land on except every fourth year.
Should a notice period be counted in days or months?: Read the wording. A three-month notice and a 90-day notice can land on different dates, and the difference has been argued in court.
Does this count weekends and holidays?: Every day is counted here. For a deadline that skips weekends, use the date calculator with the working days option.
Does this calculator store anything?: No. It runs entirely in your browser and nothing you type leaves the page.

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Date Add and Subtract Calculator

Common spans in weeks and months

Adding a month is a convention, not a sum

Months and days are not interchangeable

The dates that matter are usually about money

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Frequently asked questions

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