SQL USING CAST in a query inside a stored Procedure
You have data in approxstartdate column that cannot be converted to datetime, it’s bad data, chances are if you saw what that data is you wouldn’t know how to convert it to a datetime either.

If you are using SQL Server, take a look at try_convert which returns null if the value cannot be converted. You can use this function to find the values in your table that cannot be converted and then fix them or remove them.