Date-time groups, DTGs, are a way of writing the date and time (no, really?) as well as the time zone, usually within the military. They are used in orders, logs, and in my case a multi-time-zone flight.
DTGs are formatted as DDhhmmZMMMYY. For example, right now it’s 16:25 on 7 August 2014 in the UK – as a DTG, 071625AAUG14. The letter immediately after the time is the time zone – A being UTC+1 (British Summer Time) and Z being UTC (for most purposes, equivalent to Greenwich Mean Time).