Geohashing
Find the xkcd geohash point for a date and your graticule.
The geohashing game (from xkcd #426) turns a date into a random spot inside your graticule, the one-degree square of latitude and longitude you live in. Enter the date, your graticule, and that day's Dow Jones Industrial Average opening to get the meetup coordinate, plus the single worldwide globalhash point.
The graticule is the whole-number part of your coordinate. For N 37° 30' W 122° 30' the graticule is 37, -122.
How it works
The string date-DJIA (for example 2005-05-26-10458.68) is hashed with MD5. The 32-digit result is split into two halves; each becomes a fractional degree that is added onto your graticule's whole-number latitude and longitude.
Type the opening value exactly as published; the number is hashed character for character. The opening is only known after markets open, which is why the rule below exists.
For dates from 2008-05-27 onward, graticules east of longitude W30 use the previous trading day's DJIA opening, so geohashers in earlier time zones can compute their location without waiting for the markets. Graticules west of W30 use the same date's opening.
The same two hash fractions, stretched across the whole planet, give one global point per day shared by everyone.