How to append zeros to substring Datetime values in sql table?
Do not store dates and times as strings like nvarchar while the database has native datatypes like datetime. These are not saved as strings but in an internal binary format designed for storing this type of information. 

Such datatype saves the exact information instead of some of its various representations, allowing you to get it in different formats, making the conversion needed in this question unnecessary. More than that, it allows doing calculations and comparisons using database functions.