calculateHours.sh

#!/bin/bash

unamestr = $(uname)
if [[ "$unamestr" == 'Linux' ]]; then
    sed -i "s/[Aa]\./A/g" hours.txt
    sed -i "s/[Pp]\./P/g" hours.txt
    sed -i "s/[Mm]\./M/g" hours.txt

    # fixes a hard to notice mistake
    sed -i "s/;/:/g" hours.txt

    sed -i "s/[Aa][Mm]/ AM/g" hours.txt
    sed -i "s/[Pp][Mm]/ PM/g" hours.txt
    sed -i "s/  */ /g" hours.txt
else
    sed -i .bak 's/[Aa]\./A/g' hours.txt
    sed -i .bak 's/[Pp]\./P/g' hours.txt
    sed -i .bak 's/[Mm]\./M/g' hours.txt

    # fixes a hard to notice mistake
    sed -i .bak 's/;/:/g' hours.txt

    sed -i .bak 's/[Aa][Mm]/ AM/g' hours.txt
    sed -i .bak 's/[Pp][Mm]/ PM/g' hours.txt
    sed -i .bak 's/  */ /g' hours.txt
fi

awk '
function time_to_hours(hours, mins, am_pm) {
    return_hours = hours + (mins / 60);
    add_hours_bool = 0;

    if ((toupper(am_pm) == "AM") && (hours == 12) && (mins == 0)) {
        add_hours_bool = 1; # midnight
    } else if ((toupper(am_pm) == "PM") && (hours != 12)) {
        add_hours_bool = 1; # afternoon
    }
    if (add_hours_bool) {
        return_hours = return_hours + 12;
    }
    return return_hours;
}
function error_handling(error_message, start_hours, end_hours, current_line) {
    if ((start_hours > end_hours) || ((start_hours > 24) || (end_hours > 24))) {
        if (length(error_message) == 0) {
            error_message = "Error:";
        }
        if (start_hours > end_hours) {
            error_message = error_message "\n" "Start time before end time: " current_line;
        }
        if ((start_hours > 24) || (end_hours > 24)) {
            error_message = error_message "\n" "Time is over 24 hours " current_line;
        }
    }
    return error_message;
}
BEGIN {
    # config
    hours_decimal_precision = 2;

	total_hours = 0;
	i = 0;
	start_hours = 0;
    end_hours = 0;
    error_message = "";
    current_line = "";
}
{
    current_line = $0;
    while (match($0, /((1[0-2]|0?[1-9]):([0-5][0-9]) ([AaPp][Mm]))/)) {
        time = substr($0, RSTART, RLENGTH);

        split(time,hours_and_rest,":");
        split(hours_and_rest[2],minutes_and_am_pm," ");

        if (i % 2) {
            end_hours = time_to_hours(hours_and_rest[1], minutes_and_am_pm[1], minutes_and_am_pm[2]);
            total_hours = total_hours + (end_hours - start_hours);
            error_message = error_handling(error_message, start_hours, end_hours, current_line);

            # set to 0 for clarity
            start_hours = 0;
            end_hours = 0;
        } else {
            start_hours = time_to_hours(hours_and_rest[1], minutes_and_am_pm[1], minutes_and_am_pm[2]);
        }

        $0 = substr($0, RSTART + RLENGTH);
        i = i + 1;
    }
}
END {
    if (length(error_message) > 0) {
        print(error_message);
    }
	printf("%."hours_decimal_precision"f\n", total_hours);
}' hours.txt