Customers often like to use Excel as a means of consolidating data that needs to be imported into a database. Asking them to adhere to a consistent formatting can be a real pain. Fortunately, Excel has an option to restrict cells to a particular format.
Excel uses a badly thought out way of storing dates that also has an odd bug, http://www.robweir.com/blog/2006/10/leap-back.html|which I won’t go in to. If clients are entering data from, scratch, I tend to like them entering dates as text in the format YYYY-MM-DD. You can validate for this using the following pattern: