Scenario: In a SharePoint Designer workflow, you need to determine if a date field is blank.
Problem: SPD does not provide a direct method for comparing dates to anything other than a date. You cannot create a condition like this: "If [DateField] equals blank".
Solution: Convert the date to a string. Use string comparison to determine if the date is blank.
The following screen shots show how to do this. In this scenario, a field on an item, "Environmental Permits:First Permit Reminder Date", is submitted and the workflow fires in response.
When I tried this, I was pleasantly surprised to learn that it works. I was worried that SharePoint Designer might disallow the string assignment (Variable:StringReminderDateDate) but it did allow it.
I was also concerned that allowing it, the value might be null and either blow up the WF at runtime or maybe raise the global temperature 1/2 a degree, but those concerns were unfounded.