On em_alert_history table, for some records the "Alert Valid Time End" (vt_end) field is showing in year 8994
em_alert_history table is mainly used for Impact Calculation and Event Management dashboard.
Every time when there is a change to an em_alert record, an em_alert_history record is created.
We only record the start of each change on em_alert records, but not the end.
Thus vt_start is accurate, but for vt_end:
> on the latest em_alert_history for an alert, vt_end is year 8994
> on previous em_alert_history for the alert, vt_end equals to vt_start of next em_alert_history.
As em_alert_history is mainly for impact calculation, it's not recommended to use this table for other purpose to avoid confusions.