DayPilot is 100% type safe code, compiled with .NET Framework 2.0 with Platform target: "Any CPU". That means the resulting DLL is platform agnostic and it is possible to run it in both 32-bit and 64-bit mode.
The correct native code will be generated from the IL by the just-in-time compiler on the fly.
Whether it actually runs in 64-bit on 64-bit systems depends on the IIS settings.
64-bit only DLL
You can recompile the sources to get a 64-bit only DLL. However, it would have no other effect than preventing it from running of 32-bit systems (the setting only affects the CLR header of the DLL).