I recently switched from VB.NET to C# and found out there is no DateDiff function.
So here is a simple example of how to instead check the difference between two dates:
if (DateTime.UtcNow.Subtract(myFile.LastWriteTimeUtc).TotalSeconds > 90) myFile.Delete();
Note the use of .TotalSeconds and not .Seconds as the latter goes back to zero after each minute.