Written by Porchlight Ledger with AI assistance and checked by hand in Excel, Google Sheets and LibreOffice. Originally published on our site: Split bookings across months.

A lot of business data comes as date ranges: hotel or holiday-rental stays, subscriptions, rentals, project assignments. Reports, though, are usually monthly. If a stay runs from 29 March to 3 April and you book the whole thing in March, March looks too good and April too weak.

This post shows the formula we use to split a range across months by days (or nights), and the edge cases that tend to break it.

The core formula: overlap of two intervals

Treat each booking as a half-open interval [start, end). For a hotel-style stay, end is the check-out date, so the night of the 31st belongs to the 31st.

For a month that runs from month_start to month_end (the last day), the number of nights that fall in the month is the overlap of the two intervals:

nights_in_month = MAX(0, MIN(end, month_end + 1) - MAX(start, month_start))

With check-in in B2, check-out in C2, and the first and last day of the month in F$1 and G$1:

=MAX(0, MIN($C2, G$1 + 1) - MAX($B2, F$1))

That is the whole trick. It works unchanged in Excel 2016+, Google Sheets and LibreOffice Calc because dates are just serial numbers, so subtracting them gives a day count.

For the 29 March to 3 April stay: March gives MIN(3 Apr, 1 Apr) - MAX(29 Mar, 1 Mar) = 3 nights, April gives MIN(3 Apr, 1 May) - MAX(29 Mar, 1 Apr) = 2 nights. Total 5, which matches C2 - B2.

Allocating money by the same share

Once you have nights per month, every amount on the row can be split the same way:

share_in_month = nights_in_month / (end - start)
revenue_in_month = gross * share_in_month

Apply the same share to fees and pass-through taxes so each month's margin stays consistent.

For monthly totals, put one helper column per month next to the bookings (nights in that month), then multiply by the per-night amount and sum the column. You can squeeze it into a single array formula, but empty rows divide by zero and older Excel versions need it entered as an array formula, so the helper-column version is easier to audit. Readable beats clever in a file someone else has to trust.

Edge cases that break naive versions

1. Inclusive vs exclusive end. If your end is the last night rather than the check-out date, add 1 to it first. Mixing the two conventions is the most common off-by-one.
2. Year boundaries. A stay from 30 December to 2 January must land partly in December of one year and January of the next. The formula handles it as long as your month header holds a real date (for example 2026-12-01) rather than the text "Dec".
3. Same-day ranges. end - start = 0 means a division by zero in the share. Wrap it: IF(C2 > B2, nights / (C2 - B2), 0).
4. Text dates from CSV imports. Exports often arrive as text ("03/04/2026"), which subtracts to #VALUE! or, worse, parses with day and month swapped. Convert explicitly with DATE() from the parts rather than trusting locale parsing.
5. Leap years. Nothing to do: serial dates already know February has 29 days in 2028.

Per-event costs

Some costs happen once per range, not per day (a cleaning after a stay, a setup fee on a subscription). Either split them by the same share, or assign them to the month in which they actually happen. Both are defensible; switching between them month to month makes your trend meaningless.

Cash view vs report view

The split is for reporting income and utilisation. Cash still arrives on the payout date. Keep a separate cash view that matches your bank statement, and don't try to make one table serve both.