Are you in Singapore and your Excel defaults are set as follows? If so, try my suggestion below.
I believe that when you use the JMP Add In for Excel to open the Excel data as a JMP table, JMP is not recognizing the local English Singapore date settings. It is converting any dates beginning with 13 (which is not a month in the English US format mm/dd/yyyy) to the default mm/dd/yyyy.
IN JMP>PREFERENCES>WINDOWS SPECIFIC, examine the setting below. The locale settings for your operating system normally determine settings for number, date, and currency formats. So, if checked, try unchecking it.
Alternately, instead of using the Add-In for Excel, you can try explicitly opening the file from JMP using FILE>OPEN.
Another way to do this is to make sure your Excel column of dates is formatted as "short date". You do this in the Home tab. Highlight your column of dates. Then in the drop down menu of the "Number" section, select "Short Date". The dates will copy over to JMP nicely.