Exact day counts sound trivial until months span the gap and memory rounds things off. “About a month and a half” and “39 days” lead to different late-interest figures and different conversations.
How it is calculated
Days overdue = payment date (or today) − due date
Due date is the date the payment should have been made, as written in the agreement or schedule.
Payment date is when the money actually arrived; if it still has not, use today's date for a running count.
The convention is to count from the day after the due date: due on 1 June and paid on 2 June is 1 day overdue, and paying on the due date itself is 0 days.
Calendar months have different lengths, so the count uses actual calendar days — not months × 30.
Example
An installment of 15,000 THB was due on 1 June 2026 and remains unpaid as of 10 July 2026. Counting the 29 remaining days of June plus 10 days of July gives 39 days overdue.
That count feeds directly into a late-interest figure: if the agreement sets late interest at 10% per year, the accrual so far is 15,000 × 0.10 ÷ 365 × 39 = 160.27 THB — so bringing the account current today costs 15,160.27 THB.
Understanding your result
Quote the day count in any reminder or notice: “your payment of 15,000 THB due 1 June is now 39 days overdue” is specific, verifiable, and harder to brush off than a vague nudge.
Use the count to pick the right tone. A few days late usually merits a friendly reminder; several weeks justifies a formal late payment notice; months of silence may call for a final demand. Escalating in written, dated steps builds the record you will want if the dispute goes further.
Remember that legal deadlines for pursuing unpaid debts exist in every country and differ widely. A debt that has been outstanding for years needs local advice — the day counter tells you the age, not your options.
Frequently asked questions
Does the due date itself count as an overdue day?
By common convention, no. A payment due on the 1st and made on the 1st is on time; made on the 2nd it is one day overdue. Count starting from the day after the due date. If your agreement defines the count differently, follow the agreement.
Should I count weekends and holidays?
For measuring how late a payment is, yes — calendar days are the standard, and late interest normally accrues on every calendar day. Business-day counting only applies if the agreement explicitly says so.
How do I turn days overdue into a late-interest amount?
Multiply the overdue amount by the annual late-interest rate, divide by 365, and multiply by the days. For 15,000 THB at 10% per year, 39 days gives 15,000 × 0.10 ÷ 365 × 39 = 160.27 THB. The rate must come from your agreement and respect any local legal caps.
At how many days overdue should I send a reminder?
There is no legal trigger — it is about protecting the relationship and the money. A common pattern: a friendly message within the first week, a written reminder around two weeks, a formal notice at about a month, and a final demand after two or more months of silence. Adjust to the relationship and amount.
The borrower paid part of the amount. How do I count days now?
Track the unpaid remainder. If 5,000 THB of a 15,000 THB installment arrived on day 20, the remaining 10,000 THB continues accruing days from the original due date. Keep a dated record of every partial payment — it changes both the overdue amount and any interest calculation.
Can a debt become too old to collect?
Every country sets time limits for enforcing debts through the courts, and they vary by country and by type of debt. Written acknowledgments or partial payments can restart the clock in some places. If a debt is more than a couple of years old, check the rules where you live before deciding next steps.
Results are estimates for general information only and may differ from a lender's exact figures. They are not financial or legal advice.