2ND · DATE
Date Calculator
Count the days between two dates on the real calendar or on the 30/360 one, or roll a date forward by a number of days. The same arithmetic the BA II Plus DATE worksheet does, with the month-by-month working shown.
- Days between two dates
- Add or subtract days
- ACT and 30/360 conventions
- Working days and year fractions
Background
What a day count actually is
Two dates and the number of days between them looks like the simplest arithmetic on the calculator, and it is the one that most reliably produces two defensible answers. A hotel bills the span from 15 September to 15 March as one figure, a bond accrues interest on another, and a treasurer’s spreadsheet reports a third. None of them is wrong. They are counting under different rules, and the rules are worth knowing before the answer is used for anything.
The first rule is what a day count includes. This page, and the DATE worksheet it mirrors, count elapsed days: the later date minus the earlier one, so the start date is excluded and the end date is included. From 15 September 2026 to 15 March 2027 that is 181 days. The convention exists because interest accrues overnight — money deposited on the 15th earns nothing that day and earns for the first time on the 16th. An inclusive count of calendar days, the way a hire company bills, would be 182. That is a billing convention, not an interest one, and the difference is always exactly one day.
The second rule is which calendar to count on. ACT uses the real one, February and leap days included. 360 counts every month as 30 days and every year as 360, which turns those same six months into exactly 180 — and makes half a year worth exactly 0.500000 of a year, which is the entire reason the convention was invented. Across the span above the two differ by a single day. Across the decade to 15 September 2036 they differ by 53: 3,653 actual days against 3,600.
The formulas
How the days are counted
Four lines cover everything on this page. Only the second has an exception attached, and that exception is the source of nearly every disagreement between two people counting the same span.
DBD (ACT) = DT2 − DT1, on the real calendar
DBD (360) = 360 × (Y₂ − Y₁) + 30 × (M₂ − M₁) + (D₂ − D₁)
weeks = DBD ÷ 7
year fraction = DBD ÷ days assumed in a year
The exception is the US end-of-month rule that the 30/360 line carries. If D₁ is 31 it is treated as 30. If D₂ is 31 it is treated as 30 only when D₁ was already the 30th or the 31st. So 31 January 2026 to 31 March 2026 is 60 days on the 30/360 calendar but 59 actual days: the convention pays February the 30 days it does not have. Move the start back one day, to 30 January, and both calendars agree at 60.
One consequence catches people who split a span into pieces. Because the rule is applied to the two ends of the span rather than to every boundary inside it, 30/360 counts are not additive. Count 31 January to 31 March in two hops through 1 March and you get 31 + 30 = 61. Count it as one span and you get 60. The month-by-month table above therefore reports each period as the change in the running total rather than as a count of its own, which is why the column always adds up to the figure the worksheet reports.
-
Decide which of the three you are solving for
The worksheet holds three numbers and any two of them fix the third. Two dates give a day count; a date and a count give the other date. On the hardware you type the two you know and press CPT on the third; here the field being solved for leaves the form so it cannot be filled in by mistake.
-
Count the actual calendar days
15 September 2026 to 15 March 2027 crosses two 30-day months, three 31-day months and a 28-day February, and lands on 181. February 2027 has no leap day; 1 March 2027 to 1 March 2028 does, so that year is 366 days rather than 365.
-
Count the same span on the 30/360 calendar
360 × (2027 − 2026) + 30 × (3 − 9) + (15 − 15) = 360 − 180 + 0 = 180. Six months, exactly half a year, with no reference to how long the months really were.
-
Turn the count into weeks, working days and a year fraction
181 ÷ 7 is 25.857 weeks — 25 whole weeks and 6 days. Each whole week contributes five weekdays and two weekend days, giving 125 and 50; the six days left over run from Wednesday 16 September and add four weekdays and two weekend days, for 129 and 52. The year fractions are 0.495890 on ACT/365, 0.502778 on ACT/360 and exactly 0.500000 on 30/360.
On the hardware
The DATE worksheet, field by field
Press 2ND DATE and the worksheet opens on DT1. Three fields follow, reached with ↓, plus one setting that is toggled rather than typed. It is the shortest worksheet on the calculator and the one most often left in the wrong state.
| Field | Holds | This span |
|---|---|---|
| DT1 | The earlier date, entered MM.DDYY | 9.1526 |
| DT2 | The later date, entered MM.DDYY | 3.1527 |
| DBD | Days between the dates — elapsed, so the first day is excluded and the last included | 181 |
| ACT / 360 | The day-count convention, toggled with 2ND SET rather than typed | ACT |
Dates go in as MM.DDYY, which is the part worth rehearsing. 15 September 2026 is 9.1526, and a single-digit day keeps its leading zero: 5 March 2027 is 3.0527, because dropping the zero to 3.527 hands the worksheet a 52nd day of the month. Whichever field you want, enter the other two and press CPT on it — the calculator solves for DT1 and DT2 as readily as for DBD, so adding 90 days to a date and working back from a deadline both live here.
Two limits are worth remembering. The hardware accepts dates from 1 January 1980 through 31 December 2079 and rejects the rest, which a two-digit year makes unavoidable. And the ACT/360 setting persists after the worksheet is closed, so a count taken straight after a bond calculation is being made on whatever convention that bond used. This page has neither problem: the convention is a visible field, and any year from 1000 to 3000 is accepted — though a figure outside 1980–2079 cannot be reproduced keystroke-for-keystroke on the calculator, and the keystroke line says so when you stray outside it.
Worked example
Six months, counted two ways
The span loaded above runs from Tuesday 15 September 2026 to Monday 15 March 2027. Counted month by month, the two conventions diverge and then converge again — which is easier to see laid out than described.
| Month | Actual days | 30/360 days | Running actual | Running 30/360 |
|---|---|---|---|---|
| Sept 2026 | 16 | 16 | 16 | 16 |
| Oct 2026 | 31 | 30 | 47 | 46 |
| Nov 2026 | 30 | 30 | 77 | 76 |
| Dec 2026 | 31 | 30 | 108 | 106 |
| Jan 2027 | 31 | 30 | 139 | 136 |
| Feb 2027 | 28 | 30 | 167 | 166 |
| Mar 2027 | 14 | 14 | 181 | 180 |
The 30/360 column gives 30 days to every whole month, so it falls behind through October, December and January — three days behind by 1 February — and then hands all but one of them back in a February it insists has 30 days. The two part-months at either end are counted identically by both conventions, because the span starts and ends on the same day of the month. That is the tidy case. Start on the 31st and the end-of-month rule takes over.
What the 181 is then used for decides which of the two you want. As a span of working time it is 25.857 weeks, or 129 weekdays and 52 weekend days before any public holiday is taken off. As a fraction of a year for an interest calculation it is 0.495890 on ACT/365, 0.502778 on ACT/360 and exactly 0.500000 on 30/360. On a 5% rate applied to 100, those three fractions accrue 2.479452, 2.513889 and 2.500000 — a spread of three and a half pence per hundred, from nothing but the choice of calendar. On a hundred million of notional it is £34,437.
Troubleshooting
Five ways a day count goes wrong
Four of these produce an answer that looks perfectly reasonable, which is what makes them expensive. Only the first one errors out.
-
The date entered day-first
The worksheet reads MM.DDYY and nothing else, so 15 March 2027 is 3.1527. Typed the European way as 15.0327 it is a fifteenth month and the calculator refuses it. The version that does not error is the dangerous one: someone reaching for 12 March 2026 out of day-first habit types 12.0326, and the worksheet accepts it happily as 3 December 2026 — nine months adrift with nothing on the display to say so.
-
The convention left over from the last job
ACT and 360 survive closing the worksheet. Over this six-month span the two answers are 181 and 180, a gap small enough to pass for a rounding slip. Over the decade to 2036 it is 3,653 against 3,600, and by then whatever was built on the number has been built.
-
Counting both ends when the answer is elapsed days
DBD excludes the start date and includes the end date, because that is how interest accrues. Billing usually does not work that way: a room occupied from 15 September to 15 March is 182 days on an invoice and 181 days of interest. If you need the inclusive count, add one — the calculator will not do it for you.
-
Losing the direction of travel
Solving for DT2 rolls a date forward, and solving for DT1 works back from a deadline. A negative day count reverses either one, which is useful and easy to do by accident: 28 August 2026 plus 90 days is Thursday 26 November 2026, and minus 90 days is 30 May 2026. Check the day of the week on the answer — it is the cheapest sanity test there is.
-
Treating weekdays as working days
The 129 weekdays in this span are weekdays, not business days. No calculator can subtract your public holidays, because they depend on the country and often the city, and the settlement conventions that matter to a trade depend on both ends of it. Take the weekday figure and deduct the holidays on your own calendar.
Where to go next
Once the days are counted
A day count is rarely the answer to anything on its own. It is the input to an interest calculation, and these are the pages that take it.
- For accrued interest and a settlement price on the same two dates, with the same ACT and 30/360 choice, use the bond calculator.
- To turn a rate quoted over part of a year into an annual one, or a nominal rate into the effective rate that compares with it, use the interest conversion calculator.
- For a loan or a deposit over a whole number of periods, the TVM calculator and the amortization calculator handle the payment and the schedule.
- For flows that fall on irregular dates, discount them with the NPV calculator or find the return with the IRR calculator, using the year fractions from this page as the periods.
- To practise the 2ND DATE keystrokes exactly as they behave on the hardware, run the full BA II Plus emulator, or read the study guides.
FAQ
Date questions, answered
The eight things people ask most about counting days and the DATE worksheet.
How do I count the days between two dates on the BA II Plus?
Press 2ND DATE to open the worksheet, type the earlier date into DT1, press ↓ and type the later date into DT2, then press ↓ again to reach DBD and press CPT. Dates are entered as MM.DDYY, so 15 September 2026 is 9.1526 and 15 March 2027 is 3.1527. The worksheet answers 181 for that pair.
What is the difference between the ACT and 360 settings?
ACT counts the days that actually exist on the calendar, February and leap days included. The 360 setting counts every month as 30 days and every year as 360, which is the convention most US corporate and municipal bonds use for interest. Over the six months from 15 September 2026 to 15 March 2027 the two answers are 181 and 180. Over a decade the gap compounds: 3,653 actual days against 3,600 on the 30/360 calendar.
Does the answer include the first day, the last day, or both?
Neither convention counts both. DBD is elapsed days — the later date minus the earlier one — so the start date is excluded and the end date is included. That is the right count for interest, which accrues overnight. If you need an inclusive count of calendar days, as a hotel or a hire company would bill it, add one: 181 elapsed days from 15 September is 182 days on the calendar inclusive of both ends.
How many working days are there between two dates?
The span above holds 129 weekdays and 52 weekend days. This page reports both, worked out from the day of the week the start date falls on rather than by counting through the calendar, so the answer is instant over any span. No calculator can subtract your public holidays, because they depend on the country and sometimes the city — take the weekday figure and deduct the holidays on your own calendar.
How does the 30/360 convention handle the 31st and February?
Under the US 30/360 rule, a start date on the 31st is treated as the 30th, and an end date on the 31st is treated as the 30th only if the start date was already on the 30th or the 31st. That is why 31 January to 31 March comes out at 60 days on the 30/360 calendar but 59 actual days — the convention pays February the 30 days it never has. February month-ends are left alone: 28 February is 28 February, and the shortfall shows up as fewer days than the convention assumes.
Can I add days to a date, or work backwards from a deadline?
Yes — that is what solving for DT2 and DT1 does. Set the solve field to DT2, enter the start date and the number of days, and you get the landing date with its day of the week: 28 August 2026 plus 90 days is Thursday 26 November 2026. Solving for DT1 runs the same arithmetic backwards from a deadline, which is how a settlement or notice date is usually found. A negative number of days reverses the direction either way.
What range of dates does the DATE worksheet accept?
The BA II Plus itself accepts dates from 1 January 1980 through 31 December 2079, and rejects anything outside that window — which matters if you are checking historical bond math or a very long-dated projection. This page is not restricted to that range, so a figure it gives you outside 1980–2079 is correct but cannot be reproduced keystroke-for-keystroke on the hardware.
Are leap years handled automatically?
On the ACT setting, yes: the actual calendar is used, so 1 March 2027 to 1 March 2028 is 366 days because 2028 is a leap year, while 1 March 2026 to 1 March 2027 is 365. On the 360 setting both come out at exactly 360 — the convention has no leap day to give. A century year is only a leap year when it divides by 400, so 2000 was and 2100 will not be.
Keep going
Related calculators
Same engine, same sign convention — pick the worksheet that matches your problem.
Profit Margin Calculator
Cost, price and margin solver plus markup conversion and breakeven.
2ND · PROFIT · BRKEVNTVM Calculator
Solve for N, I/Y, PV, PMT or FV with P/Y, C/Y and BGN/END handling.
N · I/Y · PV · PMT · FVNPV Calculator
Discount up to 24 uneven cash-flow groups and read NPV, NFV and payback.
CF · NPV