Date function enhancement Request LibreOffice doc attached.

[Leslie S Satenstein]

I noted that there is no "date +%letter" that will tell me directly, the number 
of days since  January 1, 1970.  or  julian date routines


v=date +%secho (( $v / 86400 ))
17548  #the number of days since 1 jan 1970 ( a Thursday)

It would be preferable if the julian date rouitine shown below was used. It 
allows modulo 7 arithmetic to determine the day of the week. With the code 
shown incorporated as two +%? values (forward to julian and back). benefit  
easy calculation of the difference between two dates as well, the day of the 
week.  With the logic shown,     year 0000 12 31 is a Sunday.   

C code for julian to gregorian and reverse is provided below my signature.  The 
code was provided in the defunct Dr Dobbs journal by Dr. Peter Meyers (his 
paper is attached) and is in the publc domain.
 Some test results with the attached algorithms. 
juldate  1970 1 1 
Julian Day = 2440588 WeekDay=4, WeekdayISO=4="Thursday" 
Year=1970,Month=01,Day=01    (matches cal 1970 )
juldate  2018 1 17
Julian Day = 2458136 WeekDay=3, WeekdayISO=3="Wednesday" 
Year=2018,Month=01,Day=17  (modulo 7 on Julian day = 3)
Does coreutils have anything similar, so that the date function can be left 
alone?

Regards 
 Leslie
 Leslie Satenstein
Montréal Québec, Canada


The Julian day (jd) is computed from Gregorian day, month and year(d, m, y) as 
follows: 
 jd = ( 1461 * ( y + 4800 + ( m - 14 ) / 12 ) ) / 4 +
      ( 367 * ( m - 2 - 12 * ( ( m - 14 ) / 12 ) ) ) / 12 -
      ( 3 * ( ( y + 4900 + ( m - 14 ) / 12 ) / 100 ) ) / 4 +
      d - 32075
Division is to be understood as in integer arithmetic, with theremainders 
discarded. 

Converting from the Julian day to the Gregorian day is performedthus: 
    l = jd + 68569
    n = ( 4 * l ) / 146097
    l = l - ( 146097 * n + 3 ) / 4
    i = ( 4000 * ( l + 1 ) ) / 1461001 //(that's 1,461,001)
    l = l - ( 1461 * i ) / 4 + 31
    j = ( 80 * l ) / 2447
    d = l - ( 2447 * j ) / 80
    l = j / 11
    m = j + 2 - ( 12 * l )
    y = 100 * ( n - 49 ) + i + l       //(that's a lower-case L)

JulianDateExplanation.odt
Description: application/vnd.oasis.opendocument.text