fix(lit, angular): correct DateTimeInput month indexing [Issue #566]

[shantoislamdev]

Summary

Fix DateTimeInput component in both Lit and Angular renderers to use 1-indexed months (01-12) instead of 0-indexed months (00-11) for the HTML date input value.
Fixes #566

Problem

The DateTimeInput component incorrectly formatted the value attribute for <input type="date"> by using date.getMonth() directly (which is 0-indexed).

Impact:

• Browsers reject partial or invalid dates like 2025-00-01.
• The input field appears empty instead of showing the pre-filled date.
• Affects both Lit and Angular implementations.

Solution

Updated the date formatting logic in both renderers:

1. Lit: renderers/lit/src/0.8/ui/datetime-input.ts - Line: 137-138
2. Angular: renderers/angular/src/lib/catalog/datetime-input.ts - Line 97-98

Change:

- const month = this.#padNumber(date.getMonth());
+ const month = this.#padNumber(date.getMonth() + 1);

Testing

• All test passed ✅

Checklist

• I have signed the Google CLA
• My code follows the project's style guidelines
• I have tested my changes locally

[gemini-code-assist]

Code Review

This pull request correctly fixes a bug where DateTimeInput components were using 0-indexed months, causing date inputs to appear empty. The fix is applied consistently to both the Lit and Angular renderers by adding 1 to the result of date.getMonth(). The changes are clear, correct, and include explanatory comments. The code quality is good, and I have no further suggestions for improvement.

[selamw1]

While this PR fixes the primary date formatting bug, it doesn’t yet address the parsing of time-only values. JavaScript's Date constructor is unreliable when parsing 'HH:MM' strings (often resulting in Invalid Date).

To resolve this, we should add an early exit in formatInputValue. If the inputType is 'time' and the value is already a valid time string, we should return it directly to bypass the Date object logic.

Suggested Changes (to be applied to both Lit and Angular components):

// If it's a time-only input, return the value directly to avoid unreliable Date parsing
if (this.getInputType() === 'time' && /^\d{2}:\d{2}(:\d{2})?$/.test(value)) {
  return value;
}

const date = value ? new Date(value) : null;
// ... rest of the logic

What are your thoughts on this approach?