In the challenge for the "Using Formula Fields" Trailhead module, I'm using the formula, TODAY()  -  LastModifiedDate, with the type set as Number (as specified in the challenge instructions, but I receive the following syntax  error: Error: Incorrect parameter type for operator '-'. Expected Number, Date, received DateTime.

I am a total admin n00b and would appreciate any feedback on the (probably obvious) element I'm missing here. Thanks!