The date picker is a mini calendar for quickly moving between dates. It's located at the top of the left side in the control panel (browser only).
If you don't see the date picker, here's what to check:
Is the control panel collapsed? The date picker is a control panel element. If the control panel/sidebar is collapsed, tap the double arrows to expand it.
Are you on a small screen? Due to screen size limitations, the date picker will automatically be hidden to save space for other calendar details. Zooming out on your browser can bring it back into view.
Is the feature enabled? A calendar admin can turn disable the date picker feature in Settings > General Settings. Check with your calendar administrator.
control panel, date picker, date switcher, mini calendar, sidebar