You can use DATEVALUE function when you have dates which are stored as a text. It will help you to convert those textual dates into actual dates.
As you know, Excel stores a date as a serial number.
But, when a date is stored as text, even the format is correct, you can’t use that date in any kind of calculation.
In that situation, you can use DATEVALUE to convert that text into an actual date.
If you want to check whether a date is valid or not. Try to convert that date into a number. And, if you are not able to convert that date into a serial number that means it’s a text.
In the below example, I have inserted a date directly into the function by using double quotation marks.
If you skip adding these quotation marks it will return a #NAME? error in the result.
In the below example, all the dates on the left side are in textual format.
Note: If you use a date which has no month in it, it will not convert that date into a date serial number.
This function is from the list of top 100 functions with examples and from here you can also learn some useful formulas.