cal.sh

#!/bin/bash

case $# in
    1|2) ;;
    *) echo "Usage: $(basename $0) year1 [year2]" >&2; exit 1;;
esac

offset=$(($(date "+%Y") - 1950))

case $(basename $0) in
    convert-calbc)
        case $# in
            1)
                bp=$(echo "$1 1950 +pq" | dc)
                echo "cal BP = $bp age = $(($bp + $offset))"
            ;;

            2)
                bp1=$(echo "$1 1950 +pq" | dc)
                bp2=$(echo "$2 1950 +pq" | dc)
                echo cal BP = $bp1-$bp2 age = $(echo "$bp1 $bp2 + 2 / $offset + pq" | dc)
            ;;
        esac
    ;;

    convert-calbp)
        case $# in
            1)
                echo "cal BP = $1 age = $(($1 + $offset))"
            ;;

            2)
                echo cal BP = $1-$2 age = $(echo "$1 $2 + 2 / $offset + pq" | dc)
            ;;
        esac
    ;;

    *)
    ;;
esac