A B727-200 WAS DISPATCHED IN NON COMPLIANCE WITH A #3 FUEL QUANTITY INDICATOR SYS INOP AND DEFERRED FOR 3 DAYS WHICH WAS EXCEEDED BY 1 DAY.

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WHILE PREPARING TO LEAVE ZZZ ARPT TO TAKE ABC B727 TO DFW FOR THE ONLY LEG OF THE DAY; WE NOTICED THAT MEL XX-YY WAS YELLOW PLACARDED AFTER THE LAST FLT INTO MIA ON JAN/TUES/99. THE MEL STATES REPAIR IS TO BE MADE WITHIN 3 FLT DAYS. MEL GENERAL SECTION DEFINES A FLT DAY AS A 24 HR PERIOD 'MIDNIGHT TO MIDNIGHT LCL TIME;' DURING WHICH THE AFFECTED AIRPLANE IS 'INITIATED' ON AT LEAST 1 FLT. NOT COUNTING THE DAY THE ITEM WAS PLACARDED; JAN/TUES/99. ACCORDING TO OUR CALCULATIONS; THE 3-DAY CLOCK SHOULD BEGIN AT 0001 LCL ON JAN/WED/99. FIRST DAY: 0001 TO 2400 LCL ON JAN/WED/99. SECOND DAY: 0001 TO 2400 LCL ON JAN/THUR/99. THIRD DAY: 0001 TO 2400 LCL ON JAN/FRI/99. ACCORDINGLY; WE BELIEVE THE 3-DAY PERIOD ENDED AT 2400 LCL ON JAN/FRI/99. ZZZ TECHNICIAN INFORMED US THAT THE CLOCK STARTS ON JAN/WED/99 (THE DAY THE AFFECTED FLT WAS INITIATED). HOWEVER; THEIR COMPUTATION MUST HAVE BEGUN ON 2400 LCL ON JAN/WED/99 IN ORDER FOR THE ACFT TO BE LEGAL UNTIL 2400 ON JAN/THUR/99 -- THE NIGHT WE ARRIVED AT DFW. WE ARE CONCERNED THAT THIS AIRPLANE WENT DEAD AT 2400 ON JAN/FRI/99 BEFORE FLYING SEVERAL LEGS ON JAN/MON/99 INCLUDING OUR BWI-DFW LEG. ZZZ TECHNICIAN SPECIALIST AND ZZZ MOD ASSURED US THAT THE AIRPLANE WAS LEGAL UNTIL 2400 ON JAN/TUES/99; SO BASED UPON THIS WE TOOK THE FLT. WERE WE LEGAL TO DO SO? IF SO; THEN THE LANGUAGE IN OUR MEL OR ZZZ'S SHOULD BE MORE SPECIFIC SO AS TO AVOID SUCH CONFUSION IN THE FUTURE.