At times, when you are exporting your payroll to a CSV, the document will convert your numbers into date format. 


Excel is guessing those numbers are dates and coercing them into date format even though you wanted the numbers to remain as they were. 


The solution is to import the CSV instead of just opening it. That way, you can tell Excel the type of each column. There's a bit of information here:


https://support.office.com/en-us/article/Import-data-using-the-Text-Import-Wizard-40c6d5e6-41b0-4575-a54e-967bbe63a048