Wrong values (01.01.1970) are calculated when doing a Forecast on 12.3.x on some workflows tasks that were inactive according to calendar definitions. Runtime can also show something like -452172:00:00
Release : 12.3
Component : AUTOMATION ENGINE
Bug
Solution:
Update to a fix version listed below or a newer version if available.
Fix version:
Component(s): Automation.Engine
Automation.Engine 12.3.6 - Planned release in July 2021