DateTime.Compare how to check if a date is less than 30 days old?

should be

matchFound = (expiryDate - DateTime.Now).TotalDays < 30;

note the total days otherwise you'll get werid behaviour

Am I using DateTime Compare correctly?

No. Compare only offers information about the relative position of two dates: less, equal or greater. What you want is something like this:

if ((expiryDate - DateTime.Now).TotalDays < 30)
    matchFound = true;

This subtracts two DateTimes. The result is a TimeSpan object which has a TotalDays property.

Additionally, the conditional can be written directly as:

matchFound = (expiryDate - DateTime.Now).TotalDays < 30;

No if needed.

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