ARTCC CTLR AND CREW DISAGREE ON TIME ELEMENT OF A CLB CLRNC.

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I WAS WORKING AS FO AND PNF ON ACR X FLT FROM LAS-MSP. WE WERE LEVEL FLT AT FL370; DEVIATING FROM OUR RTE AROUND TSTMS. WE WERE EXPERIENCING LIGHT TURB AS WE APCHED AN AREA OF WX THAT WAS GIVING LIGHT RADAR RETURNS; SURROUNDED BY AREAS OF HVY RADAR RETURNS. FL370 WAS GOING TO PUT US RIGHT WHERE WE JUDGED THE TOPS TO BE. THE CAPT AND I DECIDED THAT WE WERE ABLE TO CLB AND FL410 WOULD KEEP US CLR OF THE BLOW-OFF WE WERE TRYING TO AVOID; IN ADDITION TO PROVIDING A SAFE MARGIN ABOVE THE WX. I CALLED ZDV AND REQUESTED FL410. THE RADIO CONVERSATION WAS AS FOLLOWS. ACR X: 'DEN; ACR X REQUEST FL410.' DENVER: 'ACR X; I WILL NEED YOU LEVEL BY XA39Z; TIME NOW XA33Z. CAN YOU MAKE IT BY XA39?' (WE WERE HVY AND WE KNEW IT WAS GOING TO BE TIGHT; BUT DETERMINED THAT WE COULD MAKE IT IN THE ALLOTTED TIME.) ACR X: 'AFFIRMATIVE.' DENVER: 'ACR X; CLB AND MAINTAIN FL410.' ACR X: 'ACR X CLB AND MAINTAIN FL410.' DENVER: 'ACR X SAY ALT.' ACR X: 'LEAVING FL407 AT THIS TIME.' DENVER: 'ACR X; SAY TIME.' ACR X: 'WE SHOW XA39 EXACTLY.' DENVER: 'YOU DIDN'T MAKE IT DID YOU?' (NO RESPONSE.) DENVER: 'ACR X; SAY ALT NOW.' ACR X: 'ACR X FL408.5.' DENVER: 'SAY TIME.' ACR X: XA40Z.' (BY XA40:15 WE WERE LEVEL AT FL410.) SO THIS RAISES THE QUESTION: DID WE NEED TO BE THERE AT EXACTLY XA30:00; OR DID WE HAVE ALL 59 SECONDS OF XA39:59 TO REACH THE ASSIGNED ALT? IF THE ANSWER IS YES; THEN SHOULDN'T WE BE REQUIRED TO TIME HACK THE SECONDS AS WELL? THE CAPT AND I BOTH CONFIRMED XA33 ON OUR INDIVIDUAL CLOCKS; HOWEVER; WE DID NOT NOTE THE SECONDS. IT IS ENTIRELY POSSIBLY THAT WE (ACR X VERSUS ZDV) WERE AS FAR AS 59 SECONDS APART ON OUR TIME HACK. IN THE PAST; CTLRS WOULD ISSUE SIMILAR CLRNCS BY GIVING AN EXACT AMOUNT OF TIME TO REACH THE CLRNC LIMIT. FOR EXAMPLE; THEY WOULD SAY 6 MINS. IT IS MY EXPERIENCE THAT THIS TYPE OF CLRNC IS FAR LESS AMBIGUOUS. WE WOULD SIMPLY START OUR CLOCKS AND SIMULTANEOUSLY START THE ALT CHANGE. THERE WAS NO QUESTIONING WHOSE CLOCK TO BE ACCURATE AND NO NEED FOR TIME HACKS. 6 MINS IS 6 MINS. IN CLOSING; IF ZDV HAD ASKED US IF WE COULD LEVEL IN 5 MINS; WE WOULD NOT HAVE ACCEPTED THE CLRNC.