I wrote the last-modified date as 01/03/2026 and it did not come out as a date. What date format does a sitemap accept?

What exactly happened

The tool looks for a date after the last comma on a line, but only treats it as a date if it starts with four digits. We tried three lines:

https://example.com/a,01/03/2026 https://example.com/b,2026-03-01 https://example.com/c,2026-02-30

• Line one: 01/03/2026 does not start with four digits, so it stayed attached to the URL, and the file got an odd URL, https://example.com/a,01/03/2026, with no date. It was not rejected, because commas and slashes are allowed in URLs, which is what makes this slip easy to miss.
• Line two: accepted, with the date 2026-03-01.
• Line three: rejected because the date after the comma is not a valid W3C date; February has no 30th.

So check the output file, not only the counts: a URL ending in a comma and digits means a date was written the wrong way round.

Formats that pass and fail

These are the tool's results for several forms:

• 2026-03-01: accepted, the simplest and most common.
• 2026-03 and plain 2026: accepted, but less precise.
• 2026-03-01T09:30:00+03:00: accepted; T separates date from time and +03:00 is the offset.
• 2026-03-01T09:30+03:00 without seconds, and 2026-03-01T09:30:00Z in UTC: accepted.
• 2026-03-01T09:30:00 with no time zone: rejected. If you give a time, you must give the zone.
• 2026-03-01 09:30 with a space instead of T: rejected.
• 2026-3-1 without leading zeros: rejected. Always use two digits for month and day.

Steps

1. Convert the dates at their source to yyyy-mm-dd. If they are in Arabic-Indic digits such as ٢٠٢٦, convert them to Western digits in the Arabic/Persian/Western digit converter first.
2. Write each date after its URL, separated by a comma, in the XML sitemap generator.
3. If every URL shares one date, type it once in 'Last-modified date for all URLs'. That field checks the date immediately: type 01/03/2026 there and an error appears and output stops until you fix it.
4. Check the 'URLs with lastmod' count above the output matches the number of URLs you gave a date.

Do not write a date you do not know

The tool never invents dates: a line with no date stays without one, and a note appears if the whole file has none. A wrong date, such as stamping the current date on every page, makes search engines trust the file less. Only write a date when you know when the page really changed.