Different values returned by SQL Server 2000 and SQL Server 2014 when returning results using for xml explicit

We have converted a database from SQL Server 2000 to SQL Server 2014. We require XML output and use T-SQL's for xml explicit option to return XML.

The timestamp value has been preserved during the conversion. i.e. A conventional query returns the timestamp value 0x00000000057A12B8 on both servers.

When using for xml explicit the SQL Server 2014 output is <Ts>AAAAAAV6Erg=</Ts> and the SQL Sever 2000 output is <Ts>91886264</Ts>

Can anybody explain the difference?