Calculate Previous Months Total

[KEO46]

I need to create a measure that documents the previouse months total number of exceptions that were submitted so I can display it in a month on month report.  The field for the exceptions, "Combine Exception'[TargetObjectId" is a text so I used the calculate count function.  I created a calendar table with a relationship to the date field, "Combine Exception'[CreatedDate ]".  But when I add the created date to the formula I am getting an error, "Function "DATEADD' expects a contiguous selection when the date column is not unique, has gaps or it contains time portion."

I have never used a calendar table before so I am not sure if the formual that I have is correct.  Below is the formula that I used for my PrevMonth field. 

PreMonth = CALCULATE(COUNT('Combine Exception'[TargetObjectId ]),DATEADD('Combine Exception'[CreatedDate ],-1,MONTH))

If you know of a video that I can watch for a first time user of a calendar table that will help me to create the measure please share it with me.

[KEO46]

Yes this was extreamly helpful.  

[amitchandak · Accepted solution]

@KEO46 , Always used a date table, marked as date tbale joined 1- Many (Single Directional) with your table

PreMonth = CALCULATE(COUNT('Combine Exception'[TargetObjectId ]),DATEADD('Date'[Date],-1,MONTH))

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