fix(server): serialize dates at UTC midnight

[LarryHu0217]

Description

Serializes Perspective date values as UTC calendar days instead of converting local midnight with mktime(). In positive-offset time zones, the previous conversion shifted a date such as 2024-01-01 to the previous UTC day. Formatted date output now uses the stored calendar value directly.

The regression test fixes the process timezone to Europe/Amsterdam and verifies both the numeric UTC-midnight value and CSV output.

Fixes #3106.

Validation

• Changed native C++ translation units compiled successfully
• test_to_format.py (91 passed, 17 skipped)
• Ruff check and format check for the changed Python test
• Post-fix runtime verification for JSON and CSV output
• git diff --check origin/master...HEAD

Tooling assistance

Codex assisted with implementation and local validation. I reviewed the resulting diff and test output.