According to the USPS Web Tools documentation, this is a sample output of the Priority Mail time in transit API:
<PriorityMailResponse>
<OriginZip>90201</OriginZip>
<DestinationZip>21114</DestinationZip>
<Days>4</Days>
</PriorityMailResponse>
The description of the Days tag just says "Numbers of days expected to deliver". Does this mean actual days or business days? Do I have to take weekends and holidays into account?