12/12/12 and 12/12/2012 should be evaluated as = dates if you are comparing them as dates and not strings.

So make sure the parameter is a date, and when comparing just make sure to always compare again the formula field and not the database field.

Before I usually compare dates against strings, i like to run the isdate() function.

if isdate({field}) then cdate({field}) else //


Regarding the else, I usually take a date from outside the date range I am using. If I am running a report for all dates from today -> today + 14, I will set the else to today - 1.