IM not gonna write the whole script for you but this should get you started:

#Current Day of Month
d=`date +%d`

#current Month
m=`date +%m`

#current Year on 4 digit format
y=`date +%Y`


TARGET=$(date -u -d '2008-07-13 00:00:00' '+%F %T.%N %Z')
CURRENT=$(date -u -d'$Y-$m-$d 00:00:00' '+%F %T.%N %Z')

#function to get the diff of 2 dates
diff () {
printf '%s' $(( $(date -u -d"$TARGET" +%s) -
$(date -u -d"$CURRENT" +%s)))
# %d = day of month.
}

# $DAYS will equal the diff of $CURRENT and $TARGET, Below numbers are 60 = Minutes per hour, 24 = Hours perday
DAYS=$(( $(diff) / 60 / 60 / 24 ))