Calendar Wiki

The Deciyear Calendar is a reform proposal which has 10 months of 4 weeks of 9 days each, summing up to 360 days. The remaining 5 days, or 6 in a leap year, are collected in a mini month called Yearend at the end of the year. These days are considered holidays or days off as are the final three days of each week (weekend). The year count and leap rule are kept from the Gregorian calendar.

The month names follow a systematic Latinate pattern:

  • Primilis
  • Secundilis
  • Tertilis
  • Quartilis
  • Quintilis
  • Sixtilis
  • Septilis
  • Octilis
  • Novilis
  • Decilis

The names for the days of the week are also regular:

  • Primiday
  • Secunday
  • Tertday
  • Quartday
  • Quintday
  • Sixtday
  • Septday
  • Octday
  • Novday


Days are subdivided with standard SI prefixes instead of hours, minutes or seconds. The inventor only mentions millidays (86.4 s ≈ 1.5 min) and microdays (86.4 ms) explicitly.


The date format is inspired by ISO and takes advantage of the partial decimalization with fixed lengths for date parts:

YYYY – Gregorian year number
YYYY.Y – month 0–9, with digit ‘A’ for Yearend
YYYY.Y:DD – day 00—35, without indication of the week
YYYY.Y:DD.D – variable length for the appended time of day to enable arbitrary precision