The calendar in your head

This won't enable you to become a perpetual calendar, but can help you determine on which day of the week a given date falls within a few months of the present or, if you know the day a specific date in the past fell, days around that time.

The basis of this is Sevens Day, the day the 7th, 14th, 21st, and 28th falls, as well as the last day of the previous month--all other dates are offset from a Sevens Day. For example, the Sevens Day for If you are looking for a specific date like 18, it is three days before 21, so back off three days from to get . It is also four days after the 14th (also ). To get one of the first days of the month, the date is the offset (no subtraction necessary). For example, 3 is three days after , a .

To look ahead a month or two, note the last day of this month will be the Sevens Day of next month (day zero). Starting with add days (you can calculate this by subtracting 28 from the current month length ), starting at to get as the Sevens Day for If you are looking for a date in add more days for the month of to get as the Sevens Day for that month.

For a past month, subtract 28 from the number of days in the prior month to get the number of days earlier in the week on which that month's Sevens Day falls. Going from to puts the Sevens Day on If looking up a date in back up

To center on other months, add ?YYYYMM to the end of the URL.

Midpoint between two dates/times Also, add or subtract to/from a date/time.
Programming day-of-week from date.
Shows day-of-week from date. Gregorian and Julian.
Misc. programs