From the wsc:
The requirement to be relieved of all duty for 24 hours in any 7
consecutive days applies to all pilots except international 4-man
crew. The 7 consecutive days are considered to be calendar days,
while the 24 hours may be any 24 consecutive hours, i.e., not necessarily
a calendar day, and may occur on a layover.

Note that this is a flight limitation. A pilot may fly for 6 days and
train or deadhead on the 7th day. A reserve pilot may be on call for
any number of consecutive days, but must have been released from
telephone responsibilities for 24 hours at some point in the past 7
days prior to reporting for domestic flight duty.