Working days and due dates: Excel interview question

Use =EOMONTH(B2,1) for “due at the end of next month” and =NETWORKDAYS(B2,C2,holidays) for the working days in between. EOMONTH jumps to the last day of a month, however long that month is, and NETWORKDAYS counts Monday to Friday with both dates included, minus the holidays you list. If a due date lands on a weekend, =WORKDAY(C2+1,-1,holidays) moves it back to the last working day.

The question

“Our invoices are due on the last day of the month after they’re issued. For each invoice, give me the due date and the number of working days the client has to pay. Leave out our office holidays.”

Sometimes it comes with a twist: “and if the due date falls on a weekend, make it the Friday before.”

What they’re really testing

• That you know dates are numbers. Subtracting one date from another gives calendar days, and that’s rarely what a business means.
• That you don’t fake a month. Adding 30 days, or building a date with day 30, breaks on every month that isn’t 30 days long.
• That you know the working-day functions. NETWORKDAYS to count, WORKDAY to move, both with a holidays range.
• Edge cases. January to February, a due date on a Sunday, a holiday inside the period.

The worked answer

Invoices are in A1:D4 and the office holidays in F2:F4. The due date in C2 is the end of the month after the issue date, which is what EOMONTH gives:

C2fx=EOMONTH(B2,1)

Row	A	B	C	D	E	F
1	Invoice	Issued	Due	Working days		Holidays
2	INV-201	12 Mar 2026	30 Apr 2026	34		3 Apr 2026
3	INV-202	30 Jan 2026	28 Feb 2026	21		6 Apr 2026
4	INV-203	20 Apr 2026	31 May 2026	29		25 May 2026

INV-202 shows why EOMONTH matters: issued on 30 January, due on 28 February, because that’s the last day of February 2026. The working days in D2 are NETWORKDAYS from the issue date to the due date, leaving out the holidays:

INV-201 has 34 working days, with Good Friday and Easter Monday (3 and 6 April) taken off. The $ signs keep the holidays range fixed when you copy the formula down.

Why “+30” is the wrong answer

The quickest answer, and the one interviewers expect a weak candidate to give, is to add 30 days. For INV-202 it lands on 1 March, a day after the real due date:

=DATE(YEAR(B3),MONTH(B3)+2,0) is a correct alternative with DATE: day 0 of a month is the last day of the month before. EOMONTH says what it means, so it’s the one to write.

The follow-up: a due date on a weekend

INV-203’s due date is 31 May 2026, a Sunday, and INV-202’s, 28 February, is a Saturday. To move INV-203’s back to the last working day of May, start one day after the due date and go one working day back with WORKDAY:

That’s Friday 29 May. Starting from the day after matters: a due date that’s already a working day stays where it is, because one working day back from the next day is the due date itself.

Common wrong answers, and why they fail

1. Subtracting the dates. C2-B2 counts every calendar day, weekends included.
2. Adding 30 days, or using day 30. Wrong for every month that isn’t 30 days long, and February catches it every year.
3. Forgetting the holidays. NETWORKDAYS without its third argument counts public holidays as working days.
4. EOMONTH with the wrong month. 0 is the end of the same month, 1 the end of the next one. Read the question twice.
5. Dates stored as text. A date typed as text, or pasted from another system, may give #VALUE! or sort and filter in the wrong order. Convert it to a real date first; the data cleaning question shows how.

Practise it: working days left in the month

A different task on new dates: the end of the start date’s own month, not the next one. We check your count, then move the start date to other months, with and without a holiday in them.

Your task: In B2, count the working days from the date in A2 to the last day of its month, both days included, leaving out the holidays in D2:D4.

Graded live

B2fx

Row	A	B	C	D
1	Start	Working days		Holidays
2	11 May 2026			25 May 2026
3				31 Aug 2026
4				25 Dec 2026

Write the formula in the bar above, then press Check or Enter.

Hint

EOMONTH(A2,0) is the last day of A2's month. Count from A2 to it with NETWORKDAYS, and give it the holidays, D2:D4, as the third argument.

Solution

=NETWORKDAYS(A2,EOMONTH(A2,0),D2:D4) Any formula that returns the same result passes.

Check

How to answer it in the interview

“Due date is EOMONTH of the issue date plus one month, so month lengths are handled. Working days are NETWORKDAYS from issue to due, with the holidays range locked. If a due date can’t be on a weekend, WORKDAY from the next day, one day back.” Mention that NETWORKDAYS counts both ends: it’s the detail interviewers check. More questions like this one: Excel interview questions.