#!/bin/bash
# daysago -- given a date in the form month/day/year, calculate how many
#     days in the past that was, factoring in leap years, etc.

# If you are on Linux, this should only be `which date`
# If you are on OS X, install coreutils with brew or from source for gdate
setglobal date = $[which date]  

proc daysInMonth
{
  match $1 {
    with 1|3|5|7|8|10|12  setglobal dim = '31'  # most common value
    with 4|6|9|11         setglobal dim = '30' 
    with 2                setglobal dim = '29'   # depending on if it's a leap year
    with *                setglobal dim = '-1'   # unknown month
  }
}

proc isleap
{
  # returns non-zero value for $leapyear if $1 was a leap year 
    setglobal leapyear = $[$date -d 12/31/$1 +%j | grep 366]
}

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

if test $Argc -ne 3  {
  echo "Usage: $[basename $0] mon day year"
  echo "  with just numerical values (ex: 7 7 1776)"
  exit 1
}

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

if test $Status -ne 0  {
  echo "Sorry, but $[basename $0] can't run without GNU date." > !2 ; exit 1
}

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

setglobal startmon = $1; setglobal startday = $2; setglobal startyear = $3

daysInMonth $startmon # sets global var dim

if test $startday -lt 0 -o $startday -gt $dim  {
  echo "Invalid: Month #$startmon only has $dim days." > !2 ; exit 1
}

if test $startmon -eq 2 -a $startday -eq 29  {
  isleap $startyear
  if test -z $leapyear  {
    echo "$startyear wasn't a leapyear, so February only had 28 days." > !2
    exit 1
  }
}

###################################
## part two: the number of days since the day you specify.
###################################

###  FIRST, DAYS LEFT IN START YEAR

# calculate the date string format for the specified starting date

setglobal startdatefmt = ""$startmon/$startday/$startyear""

setglobal calculate = ""$shExpr('10#$($date -d "12/31/$startyear" +%j)') - $shExpr('10#$($date -d $startdatefmt +%j)')""

setglobal daysleftinyear = $shExpr(' $calculate ')

###  DAYS IN INTERVENING YEARS

setglobal daysbetweenyears = '0'
setglobal tempyear = $shExpr(' $startyear + 1 ')

while test $tempyear -lt $thisyear {
  setglobal daysbetweenyears = $shExpr('$daysbetweenyears + $((10#$($date -d "12/31/$tempyear" +%j)))')
  setglobal tempyear = $shExpr(' $tempyear + 1 ')
}

###   DAYS IN CURRENT YEAR

setglobal dayofyear = $[$date +%j] # that's easy!

###   NOW ADD IT ALL UP

setglobal totaldays = $shExpr(' $((10#$daysleftinyear)) + $((10#$daysbetweenyears)) + $((10#$dayofyear)) ')

/bin/echo -n "$totaldays days have elapsed between $startmon/$startday/$startyear "
echo "and today, day $dayofyear of $thisyear."
exit 0