A 10,000-day celebration needs a day calculation because an approximate conversion to years can shift the result. Leap years accumulate across a long interval.
Enter January 1, 2000 as Starting Date, choose 10000 Days, and select After starting date. The target is May 19, 2027. For inclusive Day 10,000, choose a custom offset of 9,999 instead, giving May 18.
Check what the result represents
Use the weekday output to decide whether a gathering belongs on the target date or a nearby weekend. Keep the calculated date in the note if the actual celebration is moved.
The Remaining Days result depends on the browser's local date when you calculate. A result quoted in an old message may have a different countdown while still referring to the same target.
Extending the sequence
The presets are a finite list. After the final preset, “No later preset milestone” means you need to choose a custom target if you want another. It does not mean future dates cannot be calculated.
The form accepts custom intervals up to 1,000,000 days, but the resulting calendar date must remain within the supported years 1–9999. An excessive offset can therefore be rejected even when the number itself is within the form limit.