[bug report] datepicker date selector cannot select time

[chenshuang007]

Element UI version

2.14.1

OS/Browsers version

Win7 / Chrome 87.0.4280.88（正式版本） （64 位）

Vue version

2.6.12

Reproduction Link

https://codepen.io/pen/

Steps to reproduce

官方例子也有这样的问题 1.先选择结束时间然后点清空 2.再重新选择开始时间，这个时间会显示到结束时间的框中，其实没有选中开始时间

What is Expected?

选择即选中

What is actually happening?

期待解决

[element-bot]

Translation of this issue:\n\n### Element UI version\n2.14.1\n\n### OS/Browsers version\nWin7 / Chrome 87.0.4280.88 (official version) (64 bit)\n\n### Vue version\n2.6.12\n\n### Reproduction Link\nhttps://codepen.io/pen/\n\n### Steps to reproduce\nThe official example has the same problem\n\n1. Select the end time first, and then click clear\n2. Re select the start time, which will be displayed in the end time box. In fact, the start time is not selected\n\n### What is Expected?\nSelect as you choose\n\n### What is actually happening?\nLooking forward to solution\n\n

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.