AN OCEANIC ACR CREW REPORTS A CPDLC CLB CLRNC WITH A SPECIFIC TIME TO START THE CLB. THE CREW CLBED EARLY BECAUSE THEY SAW THE CLRNC BUT MISSED THE TIME CONSTRAINT.

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CRUISING WBOUND AT FL340; WE HAD JUST PASSED LAT/LONG 1 AT XA45Z; ENRTE TO LAT/LONG 2 (OVER THE N ATLANTIC) WE HAD BEEN REQUESTING CLRNC FOR A HIGHER CRUISING ALT FOR SOMETIME. AS WE PASSED LAT/LONG 1; 2 OPPOSITE DIRECTION ACFT PASSED OVERHEAD AND ABOUT 1 MI S OF US. (AT FL350; AND I BELIEVE; FL370) AT XB50Z A CPDLC MESSAGE (CTLR PLT DATA LNK) CAME IN FROM ZZZ OCEANIC TO CLB TO AND RPT REACHING FL370. WE ACCEPTED THIS CLRNC; CLBED TO FL370; AND AT XB53Z AN AUTO RPT WAS SENT VIA CPDLC CONFIRMING THAT WE WERE LEVEL AT FL370. AT XC07Z CPDLC MESSAGE CAME IN AND IT SAID; 'CONFIRM CLB CLRNC RECEIVED SAID AT TIME XC00 CLB.' LOOKING BACK AT THE ORIGINAL CLRNC MESSAGE; WE REALIZED THAT IT SAID; 'MAINTAIN FL340; AT XC00Z CLB TO AND MAINTAIN FL370; RPT LEVEL FL370.' WE SAW THE CLB CLRNC THAT WE WERE ANTICIPATING; BUT MISSED THE TIME TO BEGIN THE CLB. AT THIS SAME TIME; XC07Z; WE WERE CONDUCTING A CREW CHANGE WHEN A MESSAGE CAME IN TO CALL ATC; WHICH I DID UPON REACHING THE GATE. OBSERVATIONS: 1) WE SAW THE CLRNC THAT WE WERE EXPECTING AND MISSED THE CONDITIONAL STATEMENT. 2) IT MIGHT BE SAFER TO ONLY ISSUE A CLRNC WHEN IT CAN BE EXECUTED IE; NOT AT A CONDITIONAL TIME. 3) FATIGUE MAY HAVE CONTRIBUTED TO OUR MISSING THE CONDITIONAL STATEMENT. I WAS UNABLE TO SLEEP ON MY PREVIOUS REST BREAK.