Enumerations and numeric ranges: For example, the value of a particular string attribute must be a, c, or D. Another example: the value of a particular numeric must be between 1 and 500.
D {format} : the format is passed to strftime (3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation.
D {format}：括弧里的字符串被传递给strftime(3)函数，返回值填充到提示符中。如果不传参数，会按照系统默认的当地时间格式来显示。
3
D {string} - string is formatted using the strftime function.