financial excel course london - Date Format

I downloaded data from SAP into Excel. I used Functions to extract the date from the main description text cells. However for some cells, i needed to add the date in manually. The dates were inputted in different formats. Even when using the Format Cells menu and then changing the date format to match that of the format that came out of SAP, it still didn't change the format. So that when i did a pivot table, the dates didn't match. How do I get them to hold the same format?