If the "Date/Time" tab of the "Begin a Conditional (If Statement) Block" and its variants is the active tab when the action dialog is opened, the "A Value" fields will be blank.
If another tab is the active tab when opened, switching to the "Date/Time" tab will have the "A Value" fields set to the current date and time.
Switching to another tab, then back to Date/Time will not fill in those fields.
The action can be saved in this state, in which case the value will be 0001-01-01 00:00:00
This applies back at least as far as v1.8
EDIT: Fixed in v1.8.5.7