[Bug Report] DatePicker chooses errors in some cases

[jesse-li]

Element UI version

2.10.1

OS/Browsers version

windows10

Vue version

2.6.10

Reproduction Link

https://jesse-li.github.io/testCode/datepicker/test-testDatePicker.html

Steps to reproduce

请看重现链接，里面有详细的说明。

What is Expected?

选择正确的值，使用Math.round来代替Math.floor

What is actually happening?

无法选中

[element-bot]

Translation of this issue:

Element UI version

2.10.1

OS/Browsers version

Windows 10

Vue version

2.6.10

Reproduction Link

Https://jesse-li.github.io/testCode/datepicker/test-testDatePicker.html

Steps to reproduce

See the replay link, which has detailed instructions.

What is Expected?

Select the correct value and use Math. round instead of Math. floor

What is actually happening?

Unable to select

[ziyoung]

@jesse-li 写的很详细，issue 先关了，你再提交一个 pr 吧。