Are you in Singapore and your Excel defaults are set as follows? If so, try my suggestion below.
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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.
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Alternately, instead of using the Add-In for Excel, you can try explicitly opening the file from JMP using FILE>OPEN.