101-daysuntil

#!/bin/bash
# daysuntil -- Basically this is the daysago script backwards, where the
#     desired date is set as the current date and the current date is used as
#     the basis of the daysago calculation. 

# Like the previous script, use `which gdate` if you are on OS X
# If you are on Linux, use `which date`
date="`which date`"

function daysInMonth
{
  case $1 in
    1|3|5|7|8|10|12 ) dim=31 ;; # most common value
    4|6|9|11        ) dim=30 ;;
    2               ) dim=29 ;;  # depending on if it's a leap year
    *               ) dim=-1 ;;  # unknown month
  esac
}

function isleap
{
  # If specified year is a leap year, returns non-zero value for $leapyear

    leapyear=$($date -d 12/31/$1 +%j | grep 366)
}

#######################
#### MAIN BLOCK

if [ $# -ne 3 ] ; then
  echo "Usage: $(basename $0) mon day year"
  echo "  with just numerical values (ex: 1 1 2020)"
  exit 1
fi

$date --version > /dev/null 2>&1         # discard error, if any

if [ $? -ne 0 ] ; then
  echo "Sorry, but $(basename $0) can't run without GNU date." >&2
  exit 1
fi

eval $($date "+thismon=%m;thisday=%d;thisyear=%Y;dayofyear=%j")

endmon=$1; endday=$2; endyear=$3

# lots of parameter checks needed...

daysInMonth $endmon	# sets $dim variable
if [ $endday -lt 0 -o $endday -gt $dim ] ; then
  echo "Invalid: Month #$endmon only has $dim days." >&2 ; exit 1
fi

if [ $endmon -eq 2 -a $endday -eq 29 ] ; then
  isleap $endyear
  if [ -z "$leapyear" ] ; then
    echo "Invalid: $endyear wasn't a leapyear, so February only had 28 days." >&2
    exit 1
  fi
fi

if [ $endyear -lt $thisyear ] ; then
  echo "Invalid: $endmon/$endday/$endyear is prior to the current year." >&2 ; exit 1
fi

if [ $endyear -eq $thisyear -a $endmon -lt $thismon ] ; then
  echo "Invalid: $endmon/$endday/$endyear is prior to the current month." >&2 ; exit 1
fi

if [ $endyear -eq $thisyear -a $endmon -eq $thismon -a $endday -lt $thisday ] ; then
  echo "Invalid: $endmon/$endday/$endyear is prior to the current date." >&2 ; exit 1
fi

if [ $endyear -eq $thisyear -a $endmon -eq $thismon -a $endday -eq $thisday ] ; then
  echo "There are zero days between $endmon/$endday/$endyear and today." >&2 ; exit 0
fi

## if we're working with the same year, the calculation's a bit different

if [ $endyear -eq $thisyear ] ; then

  totaldays=$(( $($date -d "$endmon/$endday/$endyear" +%j) - $($date +%j) ))
  
else

  ## calculate this in chunks: starting with days left in this year

  ###  DAYS LEFT IN START YEAR

  # calculate the date string format for the specified starting date

  thisdatefmt="$thismon/$thisday/$thisyear"

  calculate="$($date -d "12/31/$thisyear" +%j) - $($date -d $thisdatefmt +%j)"

  daysleftinyear=$(( $calculate ))

  ###  DAYS IN INTERVENING YEARS

  daysbetweenyears=0
  tempyear=$(( $thisyear + 1 ))

  while [ $tempyear -lt $endyear ] ; do
    daysbetweenyears=$(( $daysbetweenyears + $($date -d "12/31/$tempyear" +%j) ))
    tempyear=$(( $tempyear + 1 ))
  done

  ###   DAYS IN END YEAR

  dayofyear=$($date --date $endmon/$endday/$endyear +%j)	# that's easy!

  ###   NOW ADD IT ALL UP

  totaldays=$(( $daysleftinyear + $daysbetweenyears + $dayofyear ))
fi

echo "There are $totaldays days until the date $endmon/$endday/$endyear."
exit 0