更好的解决方案是允许设置 null 并让验证报告错误.

Better solution is to allow set the null and let the validation reports the error.

/**
 * @var \DateTime
 * @ORM\Column(name="task_date_start", type="datetime", nullable=false)
 * @Assert\NotBlank()
 */
 private $taskDateStart;

和

 function setTaskDateStart(?\DateTime $taskDateStart) {
     $this->taskDateStart = $taskDateStart;
 }