Hey, if you have some old calendars lying around – don’t throw them out! Turns out calendars from 1991 will work for 2013. So will ones from 1985, 1974, 1963, 1957, 1946, 1935, 1929, 1918, and 2002, according to this completely useless fun fact going viral today.

DD7CC0D8EE6562964E17AA6764B8CA_h231_w308_m5_crlreZgcmThat means if you’ve got any World War I-era datebooks or disco-themed calendars on hand, those are still valuable too.

ust a word of caution: if you do decide to use the throwback calendars, make sure you don’t confuse notes like “Woodrow Wilson speech” or “Abba concert at Studio 54” with stuff you actually have to do today.

And for a more useless fun fact, here is a brain teaser.

If Christmas is on a Tuesday, so is New Year’s Day. If Christmas is on a Saturday, so is New Year’s. We all know that.

Why then, in 2011, when Christmas on a Sunday, but New Year’s Day was on a Saturday?

Answer if you don’t get it: New Year’s Day following Christmas 2011 is on Jan. 1, 2012 and is on a Sunday. New Year’s Day 2011 happened nearly 12 months before when Christmas of last year was on a Sunday.