Time part does not save date

[louis-vs]

Describe the bug
The time part does not save the provided date into the database, only the time with the date 2000-01-01.

To Reproduce

1. Edit a time part on a page
2. Set its contents to (e.g.) 2020-05-01T09:00
3. Save the page
4. The time part now shows 2000-01-01T09:00, which is the time stored in the database.

Expected behaviour
The time should save as (e.g.) 2020-05-01T09:00 - the date should be preserved.
Alternatively, and I think this would be better, the Time part should be reserved for times only, perhaps adding a third DateTime part for this purpose.

Devices:

• OS: Windows, GNU/Linux, Android
• Browser firefox, chrome
• Version latest

Additional context
I've tried some fixes, like changing the attr_json type to :datetime, but this didn't fix the issue. The problem seems to occur at the point where the part is being saved, since the server is receiving the correct data.