There's no separate "End Date" field

You won't find an End Date field to set anywhere in the Membership Level's settings - for a Fixed Membership, the end date is always calculated automatically as Start Date + Length. To change when a Fixed Membership ends, adjust its Length (or its Start Date), rather than looking for an End Date to edit.

This applies at the Membership Level. If you need to change the dates on a single Contact's existing membership, see Changing the start or end dates of a membership instead.

Renewing after a lapse resets the anniversary date

If a membership isn't renewed before it expires, renewing it afterwards (whether the member does this themselves or an admin does it for them) starts a fresh period from the new renewal date - not from where the original period would have continued. There's no automatic pro-rata adjustment for the gap.

If your organisation's constitution requires memberships to run from a fixed anniversary date regardless of when someone actually renews, you'll need to manually backdate the new membership's Start Date to the correct anniversary - see Changing the start or end dates of a membership. Bear in mind this is difficult to enforce for open, public sign-ups, since anyone can simply register again as a new member rather than renewing.