A repeated month-end boundary
October 1–31 and October 31–November 30 share October 31: one inclusive day. Whether that repeats the same charge needs a service-reference review.
Recurring invoicing
Free Billing Period Overlap Checker
Before preparing a recurring charge, compare its service dates with a prior billed period. Shared dates signal a review, but they do not prove duplicate billing when different services or approved parallel charges are involved.
What you can do here
Compare two inclusive service-date intervals and identify their shared calendar dates and day count.
Built for real invoice workflows
Resolve any repeated service coverage before preparing the next recurring invoice.
Preparing your calculator with today’s date…
Calculation notes
How the result is calculated
Both intervals include their start and end dates. Shared start = later start; shared end = earlier end. If shared end precedes shared start, overlap is zero; otherwise shared days = (shared end − shared start) + one calendar day. It checks dates only, not service categories, time zones, invoice amounts or duplicate-charge authority.
Formulas and examples reviewed October 2, 2026. Illustrative figures; no tax or contractual determination.
Adjacent periods without shared dates
October 1–31 and November 1–30 have no shared dates. No overlap does not prove that either invoice has the right amount or approval.
Questions about this calculation
Are these payment due dates?
No. They are dates of service coverage. Invoice issue dates and payment terms are separate.
Does any overlap mean a duplicate invoice?
No. Different services can legitimately cover the same dates. Review service identity, agreement and prior charges before deciding.
How to use it
1
Enter the prior service start and end dates.
2
Enter the proposed service start and end dates on the same date basis.
3
Review shared dates against the actual service and invoice references.
Why it helps
Expose shared boundary dates in inclusive periods.
Handle adjacent periods, leap days and contained intervals.
Keep date overlap separate from permission to bill.
Next step
From free tool to live invoice workflow.
Use the result when creating your invoice in InvoiceAgent. Planner setup links carry the settings they describe into a draft. Review your dates, amounts, PDF, and reminder settings before scheduling.
Related workflows
Move from the quick answer into the rest of the invoice system.
Record service coverage
Record service coverage
Link each inclusive period to its service and invoice.
Review overlap in context
Review overlap in context
Match shared dates to the actual approved charge.
Invoice templates
Invoice templates
Use the result inside invoice formats that are ready for clients and easier to automate.
Knowledge base
Knowledge base
Review the billing concepts behind the numbers before they go onto a live invoice.
Profession pages
Profession pages
See how freelancers, consultants, and agencies apply the same calculator output in different workflows.