fix datetime xls for yyyy/mm/dd hh:mm:ss

[elbert-widjaja]

Problem: yyyy/mm/dd hh:mm:ss not parsed correctly
Example: 2022/04/26 16:23:47 parsed to 2022/26/26\ 26:26:26

Cause: in xls custom format, yyyy/mm/dd hh:mm:ss was still considered as yyyy/m/d h:mm:ss in excel.
return []string{yymmdd.Format(t, wb.Formats[fNo].str)} this line is failing to parse the format correctly causing parsing error

Sample data:
t_event_short.xls

[elbert-widjaja]

@extrame may I ask for review?

[elbert-widjaja]

@extrame