At the moment, we have three constraints for validating date/time values:

• Date accepts \DateTime instances and 'Y-m-d' strings
• DateTime accepts \DateTime instances and 'Y-m-d H:i:s' strings
• Time accepts \DateTime instances and 'H:i:s' strings

I think that's a bit confusing. Usually, a value should not be a \DateTime or a string, but either of them; if it is a string, it should usually match a specific pattern.

Therefore I propose to deprecate all three constraints. The \DateTime case can be solved with the Type constraint already:

/**
 * @Assert\Type('\DateTime')
 */
private $createdAt;

The second case would be covered by a new Timestamp constraint which accepts a format as argument as supported by PHP:

/**
 * @Assert\Timestamp('Y-m-d')
 */
private $createdAt;

/**
 * @Assert\Timestamp('c')
 */ 
private $iso8601Date;

This would fix #11919 and replace #11821.