Security exception with GetCreationtime()

I have got two applications each with GetCreationTime() seemingly in very similar contexts. One throws an error and the other one doesn't. I cannot find why the bad one is bad.
I have pasted (what I hope are) the relevant bits below. Can anyone give me a clue, please ?
Security Exception Description: The application attempted to perform an operation not allowed by the security policy.  To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file. Exception Details: System.Security.SecurityException: Request for the permission of type 'System.Security.Permissions.FileIOPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. [SecurityException: Request for the permission of type 'System.Security.Permissions.FileIOPermiss ...