The significance of using the word Tattoo in the charge sheet is that the man had been granted a period of sick leave (furlough) which, as was standard practice, ended at the time of tattoo on the last day of leave. 2 July 1918 was a Tuesday. It was not good enough simply to report back in time for the first parade on the Wednesday, he was required to be back in camp the night before and before the specific time marked by tattoo. In this case, tattoo can be calculated as having been at 2130 hours, 9 30 pm, (as Donnie said) 9 days 2 and a half hours back from midnight on the 11th when he eventually returned.
A bit in the weeds perhaps. As Cally says, after his hospital stay he was allowed some leave before reporting back and he stayed away too long!
maxD