Google Sheets – Adding Date Picker with Customized Data Validation

I'm not 100% sure what the issue is, but if it is a question of how to change the range of options in data validation:

When using Data Validation, the drop downs will update automatically if you change the source-list of values that appear.

The only time you will need to re-configure the validation is if the range of the possible selections changes, or you start using a different range/sheet to validate the input cells.

Example:

options = [1, 2, 3, 4, 5]

Drop down shows:

[1, 2, 3, 4, 5]

If you want to add a value (like 6), you will need to re-do the validation because this will change the range of the options.

This can be achieved with the built-in developer tools of the browser. They can be used to input any value as a custom number format, including text with newlines and tabs.

1. Start adding a custom number format.
2. Press F12 to open the developer tools.
3. Select the console tab.
4. Paste the following text into the console:

Object.defineProperty($("[label='Custom number format']"), "value", {value: "dd/mm/yyyy\n (dddd)"})

5. Substitue "dd/mm/yyyy\n (dddd)" with your own format. Use \n in place of a newline.
6. Press enter to execute the script.
7. Apply the format. Even though the actual field contents haven't changed, our defined property will be read from when the form is submitted.

Notes

Although the original poster did ask for non-script solutions, that I believe was in reference to Google Apps Script. This answer may also be useful for others.

The reason we are selecting based on the label instead of right-clicking the field and using $0 is because it is already defined by the page. It's also unclear if the ID will change, even though an ID would mean the answer could apply to all languages without changes.

The reason we are redefining the "value" property is because the type of field used doesn't support newlines, even when set in the console.