PHILADELPHIA — The way Ernie Gross and Don Greenbaum laugh and tell jokes with the ease of old friends, it’s easy to assume the dapper octogenarians have known each other forever. In reality, they only met a few months ago. Their familiarity doesn’t come from shared memories of a childhood playground or a high school dance but a far darker place: Both men spent a single day at the Dachau concentration camp on the day its 30,000 prisoners were liberated by American GIs in 1945. Greenbaum, 87, and Gross, 83, don’t think they met that day in Dachau but nevertheless share a bond. They met after Gross, who lives in Philadelphia, saw a mention in a local newspaper last November about Greenbaum, a Philadelphia native now living in suburban Bala Cynwyd. “Ernie wanted to thank me for saving his life, quote unquote, even though there were 50,000 other men there with me,” Greenbaum said. “And we had lunch together and discussed what happened 66 years ago.” Gross, then 85 pounds after nearly a year of sickness, abuse and constant hunger, had no doubt April 29, 1945, was his last day on earth. Greenbaum, a soldier with Gen. George Patton’s Third Army 283rd Field Artillery Battalion, arrived that day at Dachau expecting to seize ammunition, clothing and food that was kept for the Nazis notorious SS forces.