http://learnenglishteens.britishcouncil.org/grammar-vocabulary/grammar-videos/prepositions-time
http://www.englisch-hilfen.de/en/exercises/structures/prepositions_at_in_on_time.htm
http://www.agendaweb.org/exercises/grammar/prepositions/time-3
http://www.englishexercises.org/makeagame/viewgame.asp?id=6259
English is fun! 