A date displayed as text needs a format mask before you can rely on it as a date. DateTime parses text into a DateTime field or formats a date into text for a report. The mask describes the order and separators, such as year-month-day. Guessing the meaning of 03/04 is unsafe when different exports use different conventions.
DateTimeAdd moves a date by the requested unit. DateTimeDiff reports a difference in the unit you specify and depends on argument order. A report across February must account for leap years, and adding one month isn't the same as adding thirty days. These exercises use ISO source dates and explicit units. Inspect both the resulting value and its field type so that a readable label doesn't replace a date needed for later calculations.