Maximum and minimum return NaN when date is timestamp

[AmyQianqianwang]

Problem

When the date column is timestamp, the maximum value and the minimum value return NaN.

Expected behavior

Return actual value.

[AmyQianqianwang]

Jeff has told me that there's something wrong with isTimestampStr pattern, and actually, it's hard to judge timestamp, so the recommendation for timestamp will be changed to integer.

So I closed this pull request.