Since months are different lengths and years start on different days, there are functions in Excel to handle these details. EDATE() increments dates by one month (whether that is a 30 or 31 day month). EOMONTH() increments a date to the end of a month (either 30 or 31). WEEKDAY() returns the day of the week (1-7) for a date. And WEEKNUM() returns the week number for a date (useful for collecting weekly sales totals).