The timeline function is designed to make searching cloud cover images by date easier.  
It works as an interactive timescale enabling the quick examination of all available images for a given time period or specific dates. In order to select the images, click on the clock icon and select the appropriate date on the timeline scale (double click) or time period (by moving the slider).
All the available images can be viewed by moving the cursor along the scale.The selected dates are featured on the desktop as the “where” parameter value and, in a visual format, in the calendar.
When selecting images by region, as displayed on the screen, changing the region leads to the selected date being discarded. To settle on a chosen image, simply click “Stop Searching”, after which working with the image, for example creating a polygon, can begin.

* the timeline function is available for images from Landsat-8 (l8), Sentinel(se) and RapidEye (re).