Shell Programming and Scripting

# returns success if date passed in is more than three months in the past
# assumes the date is in the form yyyy-mm-dd  mm and dd can be single digits.
function is_old
{
    if [[ -z $_now ]]                # "static" values set on first call
    then
        _dim=( 31 30 31 31 28 31 30 31 30 31 31 30 31 30 31 )        # days in months; yes -- 15 months!!

        _now=$(date +%s )
        _m=$(date +%Y%m )              # current month
        _y=${_m:0:4}


        if (( _y % 4 == 0 ))         # works until 2100, so keep it simple
        then
            _dim[4]=29               # leap year this year
        fi
        (( _m = ${_m:4:2} + 2 ))      # proper index into dim

    fi

    typeset tdate=$(date -d $1 +%s )   # convert date to check into seconds past epoch
    typeset cutoff=$((  _now - (86400 * ( (_dim[_m-1] + _dim[_m-2] + _dim[_m-3]))) ))

    return $(( tdate > cutoff ))
}

# simple loop to read date from stdin
while read d
do
    if is_old $d
    then
        echo "old: $d"
    else
        echo "not: $d"
    fi
done

function is_old
{
    return $((  $(date -d $1 +%s ) > $((  $(date +%s )  - (86400 * 90) ))  ))
}