To delete photos or videos from the timeline, hover over an image or video and click the circle in the top left corner to select it.
When selected, it turns into a purple circle with a white checkmark. A "Delete" button will then appear in the top bar menu.
You can select multiple files in several ways:
Click on a date to select all photos and videos from that date at once.
Select one photo or video, then scroll down, hold Shift, and click another, this will select everything in between.
Confirm the deletion in the dialogue-box that pops up, or click Cancel if you changed your mind.
If you confirm, the files will be removed from all your synced devices and all your shares and albums. Deleted photos and videos are stored in the Trash for 30 days, after which they are permanently deleted.
You can restore deleted files within this period by selecting Trash in the user menu.