Suppose, the requirement is to populate the Date field(let’s say Overtime date) on a custom object XYZ from the date time field(let’s say Job Start Date) upon record creation. Create a workflow and trigger it when the record is created on XYZ object. Workflow action will be a field update and it will be DATEVALUE(Overtime__c).

DATEVALUE – This function returns a date value from a date/time field or text expression. Additional points to note:
a. If the field referenced in the function is not a valid text or date/time field, the formula field displays #ERROR!
b.Dates and times are always calculated using the user’s time zone.

Please let me know for any comments/clarification.