XML Date: Add support for custom date format

Problem/Motivation

With an XML source where the date is not easily parsed by strtotime(), it should be possible for site builders to specify the date format to use for parsing, or at the very least support alternative date formats.

Steps to reproduce

Have an XML source where the date value is in a UK date format such as 30/06/2025 23:59:00, it should be possible for the site admin to specify the default format to parse, or at the very least support that format.

Proposed resolution

Provide support for alternative forms of dates.

Remaining tasks

Provide issue fork/patch.

User interface changes

N/A