Membership batch update: Custom date field causes Javascript date_format error

How to reproduce:

• Create a new custom field set that extends Membership
• Add a new date field in it
• Create a new profile, of type forms (for profile updates)
• Add the custom date field in it.

Now go to Find Memberships > Find all > action = batch update, select the profile, and you will see a Javascript error in the console about date_format not being defined (& broken date fields).