date: adjust arguments for two methods

Adjust arguments for better usage as below:
1) Change nextDate() to nextDate(date).
2) Change prevDate() to prevDate(date).

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

Updated date test because of the methods changed in Date module.

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