mediawiki_history includes 70 events with timestamps before 2001, all of them alterblocks events. For a full list, see this spreadsheet. In addition to the incorrect timestamps, the events all have null event_user_id and event_user_text (even though this should be filled in with the details of the blocking user) and null event_comment (even though many of the blocks had comments).
All of the users concerned seem to have a weird block of one of two types:
- the block length was a long, weirdly specific time period (like "9 decades, 9 years, 364 days, 23 hours, 49 minutes and 12 seconds" or "8 millennia, 1 century, 7 decades, 9 years, 15 hours, 13 minutes and 12 seconds")
- the block expiration was before the block date (example)