If you can post the .csv files here, someone can help you out. (or create two samples that demonstrate your data structure)
Do your customer IDs appear in multiple rows? If so, that will result in duplicated data. More info here:
You don't need to join on the date field, unless that is critical to your analysis.
Also, you said you have completed the analysis in Excel and are trying to join thereafter--I may misunderstand your point, but what you typically want to bring into Tableau is just the raw data -- and then do your analyses inside Tableau.
More info on preparing Excel files for Tableau here: Preparing Excel Files for Analysis | Tableau Software