Conversion failed error - but not converting anything and no date/time data
You didn't post your actual code, but SQL Server isn't throwing that message if there isn't an actual conversion error.

You say you aren't converting anything, but if you insert data of one data type into a column of another data type SQL Server will try to convert it implicitly.

Look at Data Type Conversion for more information:


  Implicit conversions are not visible to the user. SQL Server
  automatically converts the data from one data type to another. For
  example, when a smallint is compared to an int, the smallint is
  implicitly converted to int before the comparison proceeds.


That would happen not only with comparison but also with insert statements