Open the Photos App
Choose the "Photos" tab it it's not selected by default
Use your finger to browse to the group of photos or video clips that are organized by date
When you see the group, tap on the Select button at the top-right of the iPhone. All of the photos and/or video in the date-categorized group are selected. If you don't want specific photos or videos deleted, you can tap on the photo or video to unselect the item singularly. Items set for selection are indicated with a checkmark -- toggled on or off.
Tap the TRASH icon in the lower right to delete the bulk photos.
Confirm and Tap "Delete XX Photos" to perform irreversible delete process.