Milestones Start Time is being set to the Case record's Opened Date/Time though the Entitlement Process is set to start based on a custom date/time field
|Knowledge Article Number||000232367|
|Description||When a Milestone Start Time is set to 'Entitlement Process' and the Entitlement Process "Case enters the process" is based on a custom date/time field, some users may notice that the Milestone Start Time shows the Date/Time Opened rather the custom date/time field.
|Resolution||There is a field called "Entitlement Process Start Time" in the case object which is populated when a case is associated to an entitlement. The value is based on the "Case enters the process" of the Entitlement Process. If it is based on a custom field and the custom field is blank during the time the case was associated to the Entitlement Process, the system pulls up the Date/Time Opened field of the case instead.
When a milestone is created, it then pulls up the value populated in the "Entitlement Process Start Time" field. This is why it shows the Start Time with the Date/Time Opened rather the custom date/time field.
This behavior is expected and by design. A workaround is to create a field update that would change the value of the "Entitlement Process Start Time" based on the value of the custom date/time field.