Calculated field to capture +/- 28 days from date

HelloI need to create a calculated field that will give me 'yes' or 'no' if a date field is within +/- 28 days of another date field. So for example if a.date = 1/1/15 and b.date is 30/3/15 the calculated field will say 'no'; if the b.date was 10/1/15 it would say yes. Similarly, if the b.date was 1/11/14 it would say 'no'; if it was 10/12/14 it would say 'yes'.I appreciate I will need to write an IIF statement, probably nested, but am struggling with this, so any suggestions would be very welcome.Thanks