Why does "2025-W53" get rejected?
2025 only has 52 ISO weeks, so a week 53 in 2025 does not exist. The tool refuses rather than silently rolling it over into some other week.
Worked from a fixed input so the numbers stay put: 2025-W53-1. Try your own on the home page.
ISO year 2025 only has 52 weeks, so week 53 does not exist.
Use a week between 1 and 52 for 2025 (January 1, 2025 is a Wednesday, and 2025 is not a leap year.).
Other examples
Not a general calendar or scheduling tool, and it does not know about business fiscal-year or retail 4-4-5 week numbering, which are different systems that also call themselves "week numbers." It has no concept of time of day or timezone — it operates on calendar dates only. Not for US-style "week of the month" or non-ISO week-start conventions (e.g. Sunday-start week numbering used by some US calendars).