Drop a date on Columns and Tableau shows it at the year level as YEAR(Order Date). With a date and a measure, Automatic picks a line.
Click the + on the date pill to drill down: Tableau adds QUARTER(Order Date) beside the year. Each year becomes its own section, so the line breaks at every new year.
Right-click a date pill and each level appears twice. The first group (Year, Quarter, Month, Day) gives date parts. They're blue and discrete: every January from every year falls under one header, which suits comparing seasons. The second group, with examples like "May 2023", gives date values. They're green and continuous: each month of each year is a point on a time axis, which suits trends.
Same field, same pill text, different color, different chart. Check the color first.