Method NormalizeForStorage(DateTime)

Normalize DateTime for saving to DB.
DateTime.Unspecified and DateTime.Local are converted to UTC.
Examples:

new DateTime(2025, 11, 16) [Unspecified] → 2025-11-16 00:00:00 UTC
DateTime.Now [Local MSK 14:00] → UTC 11:00
DateTime.UtcNow [Utc] → no changes

public static DateTime NormalizeForStorage(DateTime dateTime)