NMAC WHEN A PILATUS PC12 FLT DEPARTS ON THE EXACT MIN THAT THEIR CLRNC VOID TIME WAS UP AND MEETS A BEECH BARON HEAD-ON MAKING AN APCH TO THE DEP RWY IN IMC AT 73J; SC.

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UPON DEPARTING RWY 7 AT BEAUFORT; SC;(73J) ON AN IFR CLRNC WE PASSED OVER A BEECH BARON BY 100 FT WHO WAS INBOUND ON THE GPS RWY 25 APCH (OPPOSITE DIRECTION). OUR CLRNC WAS OBTAINED THROUGH FLT SVC ON A CELL PHONE ON THE GND AND WAS ORIGINALLY ISSUED WITH A CLRNC VOID TIME OF XA40 UTC. WE INFORMED THE BRIEFER THIS WAS INADEQUATE AND AFTER SEVERAL MINS WERE ISSUED A CLRNC VOID TIME OF XA45. AFTER TKOF WE LEFT THE CTAF FREQ AND CHKED IN ON DEP. WE WERE INSTRUCTED TO REMAIN VFR AND WERE INFORMED THAT WE HAD MISSED OUR CLRNC VOID TIME. IMMEDIATELY FOLLOWING WE WERE WARNED OF TFC AT 12 O'CLOCK POS; JUST AS THE TARGET APPEARED ON OUR TCASI (TCASI; NO RA'S). WE TURNED HARD R TO THE E AS THE TFC PASSED 100 FT UNDER US. ONCE THE TFC HAD PASSED BEHIND US WE RECEIVED OUR IFR CLRNC AND CONTINUED TO SSI. AT THIS POINT WE CHKED OUR GPS CLOCK AND CONFIRMED THAT WE HAD IN FACT NOT EXCEEDED OUR CLRNC VOID TIME OF XA45. HAD WE NOT HAD TCASI; UPON BEING INFORMED TO REMAIN VFR WE WOULD MORE THAN LIKELY HAVE HAD INITIATED A DSCNT TO REMAIN VFR WHICH WOULD HAVE LIKELY LED TO A COLLISION. SUPPLEMENTAL INFO FROM ACN 544639: I WAS PIC OF THE FLT AND PNF. MY FLYING PARTNER HAD PICKED UP A VOID TIME CLRNC AND EXPLAINED WE HAD 9 MINS. WE AGREED THAT IT WOULD BE TIGHT BUT FELT WE COULD DO IT. WE STARTED THE ENG WITH 5 MINS TO GO. MINIMAL TIME TO TAXI REQUIRED AND BY MY WATCH WHICH I LATER VERIFIED TO BE 20 SECONDS SLOW OF GMT SYNC WE WERE OFF ON TIME. CALLED BEAUFORT CLBING THROUGH 500 FT AND WERE TOLD TO REMAIN VFR. I SAID 'NO SIR; WE'RE IFR.' HE SAID OUR CLRNC WAS VOID AND OUR TIME EXPIRED. THE TCASI INDICATED A TARGET -100 FT PASS RIGHT UNDER US. WE WERE 90% IMC WITH GLIMPSES OF THE GND. THEN THE CTLR CAME RIGHT BACK TO US AND SAID CLB TO 6000 FT AND PROCEED ON COURSE. WE WERE OFF DURING THE MIN BEFORE AND AT WORST ON THE MIN OF THE VOID TIME. REVIEWING OUR GPS TIMER LATER WE KNOW WE WERE OFF RIGHT ON TIME. MY AIM 5-2-4 A.1; SAYS ALL OTHER IFR TFC WILL BE SUSPENDED UNTIL THE ACFT CHKS IN OR CALLS ON THE GND/NOT OFF. IF ATC HAD FOLLOWED THIS RULE; WE WOULDN'T HAVE HAD A PROB. I LEARNED THAT OFF DURING 'THE' MIN NAMED IN THE CLRNC IS CONSIDERED VOID. MY CONCERN IS THAT VOID TIMES ARE USUALLY TIGHT AND THE CREW IS REQUIRED TO HURRY. I THINK THIS IS BASICALLY DETRIMENTAL TO SAFETY. I BELIEVE APCH MADE A MISTAKE IN AUTHORIZING OTHER IFR FLT IN THE VICINITY WITHOUT A CONFIRMATION FROM US.