Lynn, you are probably familiar with this gravesite? There is a James Hart buried at the Marietta National Cemetery with the death date as August 27th 1864, just as he has on his Confederate tombstone in Texas. Find A Grave Memorial# 3950507

There are two WBTS large cemeteries in Marietta, Georgia, one for Confederate soldiers and the other for Union soldiers and the cemeteries are sort of catty corner to each other. However upon further research the Marietta National Cemetery located at 500 Washington Avenue was used for Federal troops. Some of these burials were also reinterred from places such as Camp Cahaba in Alabama after the war. The Confederate Cemetery in Marietta, Georgia is located at 381 Powder Springs Street. Since James Hart is buried at 500 Washington Avenue I believe we can rule out this as being your ancestor and if you were using this date as the death of your ancestor we can also rule out his death date.