The timeline will show you the days, months, quarters, and years but if you prefer planning using weeks, it can also count weeks.

Displaying Week Numbers 

To show the week count, click on the timeline options ". . ." drop-down, and select week numbers.

Depending on the applied zoom, whenever the week's number can be clearly displayed, an additional row will appear showing the week count.

When you zoom out far enough, months will be replaced by weeks: