Now that you've mastered the art for dates in the 21st century, you may want to add a few twists to calculate dates in other centuries.

1.  The 20th Century
There's a simple method for calculating dates in the 1900's which avoids memorizing any new steps.  Calculating month and day of month is the same as always.  For the years and leaps, you calculate backwards from January 1, 2000 to date you're calculating.

For example, suppose you're calculating the date June 12, 1980.  First calculate the value for June 12 (3 + 12 = 15; 15 - 14 = 1).  Hang onto the 1.

Now count years.  From 1980 to 2000 there were 20 years; drop the sevens and you have 6.  Since you're counting BACKWARDS years, you SUBTRACT the 6 from your previous result.  To make it simpler, first ADD a 7 to the 1 from the previous step to get 8, then subtract the 6 from the current step and you have 2.

There were also 4 leap days between June 12, 1980 and January 1, 2000.  (The leap days were in 1984, 1988, 1992, and 1996 - remember, leap day 1980 had already passed, and leap day 2000 had not yet occurred.  The quick calculation is 20 years divided by 4 = 5; subtract one for the year 1980 and you have 4.)  Subtract 4 from the previous value (after adding a 7 to make it easier), and you have (2+7) - 4 = 5.

The result is 5 = Thursday.

2.  Other Centuries
Calculating for other centuries requires a review of the Gregorian calendar rules.  In short, there is a leap year every four years EXCEPT for turn-of-the-century years that are not divisible by 400.  Thus, the years 1600, 2000, 2400, etc. are leap years, but 1700, 1800, 1900, 2100, etc., are not leap years.

So, rather than counting backwards from the year 2000 as outlined above, you can memorize the following rule for calculating in other centuries:

• For centuries AFTER 2000, first calculate as though the date fell in the 21st century, then ADD 5 (or SUBTRACT 2) for each additional century, and ADD 1 MORE for each century divisible by 400 (i.e., for 2400, 2800, 3200, etc.). 
  
  For example, March 24, 2512 would be the same as March 24, 2012, PLUS 25 (= PLUS 4 after dropping 7's), PLUS 1.  The result for March 24, 2512 is as follows:
  • March = 2
  • 24th = 3
  • 2012 Years = 12 = 5 (after dropping 7)
  • 2012 Leaps = 4
  • Adjust for centuries from 2012 to 2512 = 5 x 5 = 25
  • Adjust for Four Hundreds = 1
  • TOTAL: 2+3+5+4+25+1 = 5 (after dropping 7's from 40) = Thursday

• For centuries PRIOR to 2000, first calculate as though the date fell in the 21st century, then ADD 2 for each additional century, and SUBTRACT 1 for each century divisible by 400, including the year 2000 (i.e., for 2000, 1600, 1200, etc.). 
  
  Thus, March 24, 1512 is calculated as follows:
  • March = 2
  • 24th = 3
  • 2012 Years = 12 = 5 (after dropping 7)
  • 2012 Leaps = 4
  • Adjust for centuries from 2012 to 1512 = 5 x 2 = 10
  • Adjust for Four Hundreds = -2
  • TOTAL: 2+3+5+4+10-2 = 1 (after dropping 7's) = Sunday