Mathematics: Calenders?

The calendar repeats every 400 years, and there are 14 separate calendars to choose from (7 of the 365-day calendars that each start on a different day of the week, and 7 of the 366-day calendars that start of a different day of the week).

Using that knowledge, you can generate 14 separate calendars and give them names like A, B, C, D, ... , L , M , N

Then, you can generate a table that will tell you what calendar to use for any year that is k + 400x (for instance, 2009 would be 9 + 400*5, so we'd use the calendar on spot 9).

So, all you'd really have to do is memorize 14 separate calendars (easier to do than it sounds), and then memorize a table of 400 values (and there's a 28-year repetitive cycle that emerges for most of the table, so that's easy to do), and then you can calculate the value of any day in history (as long as it uses our calendar system).

There are plenty of manual tools that do this (William Sidis, who was arguably one of the most intelligent men ever to live, developed a perpetual calendar in the early 20th century), so you could always make one of those.