Sources stay with the claims they support.
Pure date and time arithmetic does not need an external citation. Named observance pages are different: when a date has documented context, the underlying source links are shown on that event page.
Source-backed observances
The observance section currently publishes fixed annual dates that have been selected from a reviewed source-backed dataset. The source list on each event page is the place to inspect the evidence behind the date context.
Corrections
If an event date or source changes, the reviewed observance record should be corrected before the public countdown is updated. Calculator output does not override source evidence.