PowerBI DAX Measure To give a distinct count in last 30 days

Question

I'm new to PowerBI/DAX and can't seem to figure out how to make a measure that provides me a separate count of consumers that signed a form in the previous 30 days. I have a customer ID field (Cust ID), as well as the date the form was signed (FormSignDTS). This is how I went about it.

Last 30 Days = 
   VAR _TodaysDate = Today()
   VAR _StartDate = dateadd (Cutomer[FormSignedDTS], -30, Day)
  return 
  calculate (distinctcount (Customer[CustID]), keepfilters (Customer[FormSignedDTS] >= _StartDate))

But it got the following error:

 "a date column containing duplicate dates was specified in the call to function dateadd"

Any assistance you may provide will be highly appreciated! Thanks