I've read it in the context of military history: "D-day was to be 1st April, H-hour 0600". I've never heard it used.

I thought this was too hard, and not interesting enough to persevere with. I probably spent an hour or two over a couple of days getting 7 answers - one of which was wrong - 'Nerve Gas' is much better than my answer! Those undefined ones are difficult - I resorted to guessing answers - Plan B say - and seeing if there was an appropriate light. I might have got Generation X that way - but never X Generation.

I like the alphabetic puzzles where there are no clue numbers, or double puzzles where you don't know which one a clue applies to. They're more interesting - I stick at them and can usually finish them.