Error converting data type nvarchar to datetime.

I think this is related to the format (date format) of the string you are passing. You might need to check the culture settings of your server (DB) and the client machine (from where you have sent the date string).
This link provides you a list of different styles that can be used in date conversions in T-Sql. I also suggest to debug your code and check what string you are passing to the server, take that and convert it to date in SSMS. That would give you a better idea of your problem.