I'm assuming you store these things as VARCHARs and don't validate the input. I'm afraid the day is not so good for you - there is no easy solution for this.
If you don't have (and cannot buy) any of the off-the-shelf data cleansing tools, you would probably need to "roll your own" function with pattern matching to parse all these various date strings and convert them to a proper DATE datatype.
I thought that some more information(or data) need in the "Employes" table.
Because, there are two possible format for each sample data except first one, like...
If you can list up all possible formats and you can specify one format for each data in the table, it would be not so difficult to write a function.