Looking through the records there are several dates showing Allan joining the 22nd Battalion. The first date Allan joined the 22nd Battalion would be on 20 Sep 1915, then it looks like he became ill and was transferred to the Admin unit and Hospital. He was then reassigned to the 22nd Battalion on 4 Dec 1915, once he finally made it to the Gallipoli Peninsula. I did a search through Fold3 and found information if you would like it let me know and I can email you the file. I hope this helps you.