Day-trips are trips that a flight crewmember works that do not include an overnight (i.e. layover).  Day-trips generally do not allow flight crewmembers to write off their M&IE expenses on those days because they do not involve a legitimate rest period according to the rules spelled out in IRS Publication 463.  In essence, the way Publication 463 spells out the M&IE deduction is that crewmembers can write of M&IE expenses when the travel away from their tax home AND stay away long enough that a rest period is necessary.  The rest period does not have anything to do with FAA rest requirements.  The rest requirement that the IRS is referring to is described in IRS Publication 463.

The EZPERDIEM.COM Per Diem Calculator ^TM automatically prevents a crewmember from entering day trips.  In the rare case of a crewmember having a trip that left early in the morning, returning late that evening, and meeting the rest requirements of IRS Publication 463, the crewmember could simply enter that trip as a two-day trip.