A B727-200 INCURRED A DELAY FOR INTERP OF THE MEL ON 3 DAY DEFERRALS AND WHEN THE PERIOD BEGINS AND ENDS.

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AFTER REVIEWING LOGBOOK AND MEL; MY CREW AND I CAME TO THE CONCLUSION THAT THE ACFT WAS NOT LEGAL FOR DISPATCH ACCORDING TO MEL AND LOGBOOK ENTRIES. THE ACFT WAS BOARDED AND 30 MINS PAST DEP TIME DUE TO LATE ACFT ARR AND MAINT ON BOARD. I CALLED MAINT TECHNICIAN TO VERIFY MY INTERP OF MEL AND SHOULD THE ACFT HAVE BEEN REMOVED FROM SVC DUE TO THE PLACARDED ITEM BEING A CATEGORY 'A' REPAIR ITEM AND 3 FLT DAYS HAVE ELAPSED FROM THE DATE LISTED ON THE PLACARD. AFTER A WHILE; I HAD A MAINT CHIEF NEXT TO ME AND A MAINT TECHNICIAN SUPVR ON THE PHONE EXPLAINING TO ME THAT THE PLACARD WAS NOT IN EFFECT DUE TO ACFT BEING OUT OF SVC AND THE 3 FLT DAYS WERE BEING COUNTED AS A MONDAY; A TUESDAY; AND A THURSDAY. WITH ANXIOUS PAX AND AGENT; I CONVINCED MYSELF I WAS MISSING SOMETHING AND THEY MUST BE RIGHT. AFTER ALL; I WOULD BE THE THIRD CAPT TO ACCEPT ACFT THAT DAY. I INITIALED LOGBOOK AND HAD AN UNEVENTFUL FLT TO MCI. ENRTE; I AGAIN REVIEWED BOTH THE LOGBOOK AND MEL AND FURTHER DISCUSSED THIS WITH THE FO AND FE. WE AGAIN ARRIVED AT OUR ORIGINAL CONCLUSION -- THAT THE ACFT SHOULD NOT HAVE BEEN IN SVC UNLESS THAT ITEM HAD BEEN REPAIRED. IF MAINT TECHNICIAN IS CORRECT; THEN I HAVE NO REF TO VERIFY THEIR INTERP OF WHEN THE 3 FLT DAYS SHOULD BEGIN.