date: keep use uniform argument for methods

Change argument for setCurDate() to use uniform argument 'date'

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>
test: add tests for date module

Add more validations for both isFirstDate() and isLastDate().

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>