SSIS Script - System.FormatException: String was not recognized as a valid DateTime

I am importing data from CSV file to SQL using SSIS. The SSIS job has been running perfectly fine for last few months but suddenly it gives error message while running a script. The SSIS job is not changed at all.

It gives error while running a script which calculates maximum date from three date.

I am not sure what is causing the error as it's been running fine always. Error is as below.

[Script Component [7437]] Error: System.FormatException: String was not recognized as a valid DateTime.
   at Microsoft.SqlServer.Dts.Pipeline.ScriptComponentHost.HandleUserException(Exception e)
   at Microsoft.SqlServer.Dts.Pipeline.ScriptComponentHost.ProcessInput(Int32 inputID, PipelineBuffer buffer)
   at Microsoft.SqlServer.Dts.Pipeline.ManagedComponentHost.HostProcessInput(IDTSManagedComponentWrapper100 wrapper, Int32 inputID, IDTSBuffer100 pDTSBuffer, IntPtr bufferWirePacket)