# Ordinal date

Date 2020-09-23 2020-267

An ordinal date is a calendar date typically consisting of a year and a day of the year ranging between 1 and 366 (starting on January 1), though year may sometimes be omitted. The two numbers can be formatted as YYYY-DDD to comply with the ISO 8601 ordinal date format.

## Calculation

Computation of the ordinal date within a year is part of calculating the ordinal date throughout the years from a reference date, such as the Julian date. It is also part of calculating the day of the week, though for this purpose modulo-7 simplifications can be made.

For these purposes, it is convenient to count January and February as month 13 and 14 of the previous year, for two reasons: the shortness of February and its variable length. In that case, the date counted from 1 March is given by

$\lfloor 30.6(m+1)\rfloor +d-122$ which can also be written

$\left\lfloor 30.6m-91.4\right\rfloor +d$ or

$\left\lfloor 30.6(m-3)+0.4\right\rfloor +d$ with m the month number and d the date. $\left\lfloor x\right\rfloor$ is the floor function.

The formula reflects the fact that any five consecutive months in the range March–January have a total length of 153 days, due to a fixed pattern 31–30–31–30–31 repeating itself twice.

"Doomsday" properties:

For $m=2n$ and $d=m$ we get

$\left\lfloor 63.2n-91.4\right\rfloor$ giving consecutive differences of 63 (9 weeks) for n = 2, 3, 4, 5, and 6, i.e., between 4/4, 6/6, 8/8, 10/10, and 12/12.

For $m=2n+1$ and $d=m+4$ we get

$\left\lfloor 63.2n-56+0.2\right\rfloor$ and with m and d interchanged

$\left\lfloor 63.2n-56+119-0.4\right\rfloor$ giving a difference of 119 (17 weeks) for n = 2 (difference between 5/9 and 9/5), and also for n = 3 (difference between 7/11 and 11/7).

The ordinal date from 1 January is:

• for January: d
• for February: d + 31
• for the other months: the ordinal date from 1 March plus 59, or 60 in a leap year

or equivalently, the ordinal date from 1 March of the previous year (for which the formula above can be used) minus 306.

### Modulo 7

Again counting January and February as month 13 and 14 of the previous year, the date counted from 1 March is modulo 7 equal to

$\left\lfloor 2.6m-0.4\right\rfloor +d$ with m the month number and d the date.

Calculation can be done starting with January 1 mathematically without if statements if we take advantage of min and max algebraic logic
MAX is $1/2*(a+b+\left|a-b\right|)$ MIN is $1/2*(a+b-\left|a-b\right|)$ provided the month(m) day(d) and year(y)
$(MAX(0,MIN(1,m-1))*31)+$ //if Jan is a full month
$(MAX(0,MIN(1,m-2))*28)+$ //if Feb is a full month
$(MAX(0,MIN(1,m-3))*31)+$ //if Mar is a full month
$(MAX(0,MIN(1,m-4))*30)+$ //if Apr is a full month
$(MAX(0,MIN(1,m-5))*31)+$ //if May is a full month
$(MAX(0,MIN(1,m-6))*30)+$ //if June is a full month
$(MAX(0,MIN(1,m-7))*31)+$ //if July is a full month
$(MAX(0,MIN(1,m-8))*31)+$ //if Aug is a full month
$(MAX(0,MIN(1,m-9))*30)+$ //if Sept is a full month
$(MAX(0,MIN(1,m-10))*31)+$ //if Oct is a full month
$(MAX(0,MIN(1,m-11))*30)+$ //if Nov is a full month
$d+$ //days of current month
$(((INT((y)/4)-INT((y)/100)+INT((y)/400))$ //leap year logic
$-(INT((y-1)/4)-INT((y-1)/100)+INT((y-1)/400)))*MAX(0,MIN(1,m-2)))$ //only count a leap year if date is >=3rd month //leap year logic

example of Aug 24th 2016 is $31+28+31+30+31+30+31+0+0+0+0+24+1$ ## Table

 To the day of i Add Leap years Algorithm 13Jan 14Feb 3Mar 4Apr 5May 6Jun 7Jul 8Aug 9Sep 10Oct 11Nov 12Dec 0 31 59 90 120 151 181 212 243 273 304 334 3 0 31 60 91 121 152 182 213 244 274 305 335 2 $30(m-1)+\left\lfloor 0.6(m+1)\right\rfloor -i$ For example, the ordinal date of April 15 is 90 + 15 = 105 in a common year, and 91 + 15 = 106 in a leap year.

## Month–day

The number of the month and date is given by

$m=\left\lfloor od/30\right\rfloor +1$ $d=mod(od,30)+i-\left\lfloor 0.6(m+1)\right\rfloor$ the term $mod(od,30)$ can also be replaced by $od-30(m-1)$ with $od$ the ordinal date.

• Day 100 of a common year:
$m=\left\lfloor 100/30\right\rfloor +1=4$ $d=mod(100,30)+3-\left\lfloor 0.6(4+1)\right\rfloor =10+3-3=10$ April 10.
• Day 200 of a common year:
$m=\left\lfloor 200/30\right\rfloor +1=7$ $d=mod(200,30)+3-\left\lfloor 0.6(7+1)\right\rfloor =20+3-4=19$ July 19.
• Day 300 of a leap year:
$m=\left\lfloor 300/30\right\rfloor +1=11$ $d=mod(300,30)+2-\left\lfloor 0.6(11+1)\right\rfloor =0+2-7=-5$ November -5 = October 26 (31 - 5).