1.用户在终端执行程序后，在当前的终端窗口中显示当前的年、月、日
2.显示日历时是以表格形式显示：
*　　　　表格的标题是年月
*　　　　表头是星期，并且是从星期日开始
*　　　　显示日期时，如果当前月份的1日不是星期日，则用上个有的末尾几日补齐表格。(Program function:
* 1. users display the current year, month, day in the current terminal window after the terminal executes the program
* 2. displays calendar when it is displayed in tabular form:
* the title of the table is year and month
* the header is week, and it starts on Sunday
When the date is displayed, if the 1 day of the current month is not Sunday, the last few days are used to fill the table.
Similarly, if the end date of the current month is not Saturday, it will be filled with the beginning of next month.
*
* 3. ordinary dates are displayed in white, the current date is displayed in green, red on Saturday and Sunday, and the date used to fill the form is shown in gray.
* 4. users use left and right keys to control the month's increase and decrease, and the up and down keys control the year's increase and decrease.)