Digit sum

A digit sum is a single pass. Add the digits, report the answer, stop. The result can be any size at all, and for anything longer than a couple of digits it will usually have more than one digit itself. Reduction, by contrast, is the same operation run in a loop until it can go no further. Sources that use the two terms interchangeably are the main reason people get different answers from what looks like the same instruction.

This site's digit sum strips every character that is not a digit before adding, which is why dates can be fed to it in ISO form without special handling. The string 1990-07-16 and the string 19900716 both give 33, because the hyphens contribute nothing. That one behaviour is doing quiet work in several calculations, including the personal day number, which adds the digit sum of a birth date to the digit sum of a calendar date before any reduction happens.

The order matters more than it looks. Summing first and reducing once at the end is not always the same as reducing each part and then adding, because master numbers can be preserved at one stage and not at another. The Life Path on this site reduces month, day and year separately for exactly that reason, and it can give a different answer from summing every digit of the date in one go.