Full Date Formula
day + month + year + century − leap → mod 7
0=Sun 1=Mon 2=Tue 3=Wed 4=Thu 5=Fri 6=Sat
Leap adj: −1 if leap year AND Jan or Feb
Month Codes
| Jan | 6 | Feb | 2 | Mar | 2 | Apr | 5 |
| May | 0 | Jun | 3 | Jul | 5 | Aug | 1 |
| Sep | 4 | Oct | 6 | Nov | 2 | Dec | 4 |
Month Offsets (days elapsed before month)
| Jan | 0 | Feb | 31 | Mar | 59 | Apr | 90 |
| May | 120 | Jun | 151 | Jul | 181 | Aug | 212 |
| Sep | 243 | Oct | 273 | Nov | 304 | Dec | 334 |
Days in Each Month
| Jan | 31 | Feb | 28* | Mar | 31 | Apr | 30 |
| May | 31 | Jun | 30 | Jul | 31 | Aug | 31 |
| Sep | 30 | Oct | 31 | Nov | 30 | Dec | 31 |
*Feb=29 in leap years
Century Offset
Year Code: (YY + ⌊YY÷4⌋) mod 7
'97 → (97+24)%7 = 2
7× Table
| 12wk | 84 | 13wk | 91 | 14wk | 98 | 15wk | 105 |
| 16wk | 112 | 17wk | 119 | 26wk | 182 | 52wk | 364 |
Quick Reference
Add ×7s — base + multiple of 7 (weekday unchanged)
7× Table — N weeks × 7
Large Mod 7 — divide, keep remainder
Day of Year — offset[m] + day (+1 if leap after Feb)
Nth Weekday — 1st occurrence + 7×(n−1)
Cross-Month — add days, roll over boundaries
Deadline — due date → full date formula
Day Count — days left + full months + end days
Rel. Weekday — (target−today+7)%7, use 7 if 0