In a previous blog, I implemented a routine to convert a Gregorian date into a Juliane day number to allow simple math and comparisons on dates.
This was pretty cool but it is less useful if you cannot convert back into a user friendly version of the date.
The conversion routines in the wikipedia entry also cover converting back to a Gregorian day, month and year. I have also implemented a routine to calculate the day of the week.
#!/bin/bash
# https://en.wikipedia.org/wiki/Julian_day#Converting_Julian_or_Gregorian_calendar_date_to_Julian_day_number
twodig()
{
val=$1
retval=$val
if [ $val -lt 10 ]
then
retval=`echo 0${val} `
fi
echo $retval
}
# div = floor(dividend / divisor)
div()
{
echo $(( $1 / $2 ))
}
# quotient = dividend / divisor
modulo()
{
echo $(($1 % $2))
}
# the lowest whole number
floor()
{
X=$1
echo ${X%.*}
}
# day of week name
dayofweek()
{
expr=$1
case $expr in
1) retval=mon;;
2) retval=tue;;
3) retval=wed;;
4) retval=thr;;
5) retval=fri;;
6) retval=sat;;
7) retval=sun;;
esac
echo $retval
}
# ordnal number of day of week
dayofweekIdx()
{
idx=`modulo $1 7`
idx=`echo "$idx + 1" | bc -l`
echo $idx
}
#####################################################################################################
# calculate the actual gregorian day/month/year from the julian value #
#####################################################################################################
getGregorian()
{
J=$1
y=4716
v=3
j=1401
u=5
m=2
s=153
n=12
w=2
r=4
B=274277
p=1461
C="(−38)"
# 1.
#
# f = jd + j + (( ( 4 * J + B) div 146097) * 3) div 4 + C
#echo "f = $J + $j + (( ( 4 * $J + $B) div 146097) * 3) div 4 + $C"
tmp=`echo "4 * $J + $B" | bc -l`
tmp=`div $tmp 146097`
tmp=`echo "$tmp * 3" | bc -l`
tmp=`div $tmp 4`
tmp=`echo "$J + $j + $tmp - 38" | bc -l`
f=$tmp
#echo f $f
# 2.
#
# e = r * f + v
#echo "e= $r * $f + $v "
e=`echo "$r * $f + $v " | bc -l`
#echo e $e
# 3.
#
# g = mod (e,p) div r
#echo "g = mod ($e,$p) div $r"
g=`modulo $e $p`
g=`div $g $r`
#echo g $g
# 4.
#
# h = u * g + w
#echo "h = $u * $g + $w"
h=`echo "$u * $g + $w" | bc -l`
#echo h $h
# 5.
#
# D = (mod(h,s)) div u + 1
#echo "D = (mod($h,$s)) div $u + 1"
D=`modulo $h $s`
D=`div $D $u`
D=`expr $D + 1`
D=`twodig $D`
#echo D $D
# 6.
#
# M = mod(h div s + m, n) + 1
#echo "M = mod($h div $s + $m,$n) + 1"
M=`div $h $s`
M=`expr $M + $m`
M=`modulo $M $n`
M=`expr $M + 1`
M=`twodig $M`
#echo M $M
# 7.
#
# Y = (e div p) - y + (n + m - M) div n
#echo "Y = ($e div $p) - $y + ($n + $m - $M) div $n"
Y=`expr $n + $m - $M`
Y=`div $Y $n`
tmp=`div $e $p`
Y=`expr $tmp - $y + $Y`
#echo Y $Y
echo $Y$M$D
}
JD=2451545
USERDATE=`getGregorian $JD`
IDX=`dayofweekIdx $JD`
DAYOFWEEK=`dayofweek $IDX`
echo $USERDATE is a $DAYOFWEEK
idx=0
while [ $idx -lt 15 ]
do
JD=$((2451554 + $idx))
IDX=`dayofweekIdx $JD`
DAYOFWEEK=`dayofweek $IDX`
echo $JD $IDX $DAYOFWEEK
idx=$(($idx + 1))
done
