WHILE ON A PLT'S DISCRETION CLRNC; AN ACR DC10 FLC FAILED TO INITIATE A CLB FROM FL290 TO MEET A XING RESTR FOR MOATT; AT FL310. THE PNF HAD LEFT THE COCKPIT TO ATTEND TO PHYSIOLOGICAL NEEDS AND DISCOVERED THE PF'S FAILURE TO CLB TO FL310 WHEN HE RETURNED. THE MONCTON ARTCC RADAR CTLR INTERVENED WHEN THE FLT WAS OVER MOATT.

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AROUND 129-140 NM SW OF MOATT ON A NE HDG TO THE FIX; MONCTON CTR CLRED OUR FLT TO CLB TO FL310 AT PLT'S DISCRETION; BUT TO CROSS MOATT AT FL310. I WAS THE PNF WHILE THE CAPT WAS THE PF. THE CAPT; HEARING THE MOATT XING RESTR; PUT FL310 IN THE DC10'S FLT GUIDANCE SYS MODE ANNUNCIATOR. SOON AFTER THIS EVENT; BUT PRIOR TO MOATT; I HAD TO GO TO THE LAVATORY. UPON LEAVING THE FLT DECK; I FOUND THE RESTROOMS ADJACENT TO THE COCKPIT IN USE. I HAD TO WAIT A FEW MINS BEFORE I COULD ENTER AND USE THE BATHROOM. UPON RETURNING TO MY FLT POS; I STARTED TO FILL OUT THE PAPERWORK FOR THE MOATT FIX RPT. WE WERE ABOUT 16 MI FROM MOATT AT THIS TIME (STILL SW OF THE FIX ON A NE HDG). AT THIS TIME I ALSO NOTICED WE WERE STILL AT FL290 AND HAD NOT CLBED TO FL310; OUR MOATT XING RESTR. WHILE WE WERE OVER MOATT; MONCTON CTR CALLED TO REMIND US WE HAD NOT CLBED; AND DIRECTED US TO IMMEDIATELY CLB TO FL310. ON HINDSIGHT; I SHOULD HAVE REMINDED THE CAPT OF THE XING RESTR WHEN I LEFT THE FLT DECK; AND I SHOULD HAVE IMMEDIATELY LOOKED AT THE ALTIMETER; AS A DOUBLE- CHK; UPON MY RETURN. I REALIZED OUR ERROR 1-2 MINS AFTER MY RETURN.