Hmm. Had the book in my hand when I said 45. That was the time mentioned in the chapter callled "Mustangs". If it was Summer 44 he'd probably have bumped into the Russian Shuttle run 51 escorts
One of the Mustang fights he talks about was when the Mustangs and Russian fighters got into it after the 109s dove through.
Note that the first part of the chapter is a flashback from when Hartmann is reassigned from Me 262 training back to I/JG52 in Czechoslovakia... which would make that around March/April.
'Erich's squadron was ordered to operate from a grass strip at Zilistea, a few minutes' flying time from Ploesti. The date was 23 June 1944'
Then it comes back to I/JG52 in Czechoslovakia after his encounter with the Mustangs that forced him to bail out.
'Erich reviewed these five battles with the P-51 Mustangs, which had taken place in the spring of 1944, as he droned through his air journey back to Czechoslovakia. Almost a year had passed since he had battled the Americans in Rumania. By now, they would certainly be stronger.'