Century: BC & AD

2009 AD = Anno (After) Domini (Lord)
800 BC = Before Christ

Calendar Math

Mental quick calendar days:
Reference Date: last day in Feb (doomsday) 28/2/2013= Thursday
The following days are all Thursday: 4/4, 6/6, 8/8, 10/10, 12/12
The odd Thursday:
5/9, 9/5 (9 am to 5 pm work)
7/11, 11/7 (Seven-eleven Shop)
7/3 (7+3 = 10)

Quick applications:

Singapore National day 9/8/2013 = Fri (8/8 + 1)

Xmas 26/12/2013  = Thurs (12/12 +2*7)