主题：日期类型值为空的判断

Dim partyTime       As Date = e.Form.Controls("partyTime").Value
Dim retireTime      As Date = e.Form.Controls("retireTime").Value
Dim workdate        As Date   = e.Form.Controls("workdate").Value
Dim birth           As Date = e.Form.Controls("birth").Value

如何判断出，对应的时间输入框中输入的时间为空