Cells contain different types of data — dates, quantities, text, and more.
Take dates. One cell, one date — but a date carries a lot of information inside it. Break it down and you get: year, month, day. From the month alone you can derive: first half or second half of the year, fiscal quarter. And from the date itself, you can calculate the day of the week.
When you want to group data by year, by month, by day, or by weekday, you need to extract those values first.
Let's create columns for t