You must specify a valid date within the range of 1/1/1900 and 12/31/8900
What causes it and how to fix it:
- Created/Modified on a document library won't stick — and the API says 200 while ignoring youA plain MERGE reports 204 and SharePoint overwrites both with "now"; ValidateUpdateListItem + bNewDocumentUpdate is the only path that holds, it wants the web's locale format (the error message tells you which), reads it in the web's regional time zone rather than the browser's, truncates to the minute even when you send seconds, and drops Author without a word unless all four author/date fields arrive togetherCause and fix →