In the usual CS program, it is often used to enable the user to enter time. In order to prevent user input format errors, we generally use time to select control; but use this control there is a problem: it is difficult to represent null values, in order to solve This problem, we sometimes use the check attribute to determine if the user enters valid, and it is necessary to enter, otherwise it is replaced with one default. However, these are not the user really want. After all, more times, the user thinks that if I don't enter anything, it should be processed as a null value. In order to solve this problem, the brain has been ramped, and a control has been developed to complete the time null value, as shown in the following figure, it can complete the most of the functionality of the normal time to select the control, and can express null values perfect.
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