See Date-Time Functions and Formats for information about date time formats. Depending on what you want to do with them (your then statement), you may want to transform the dates first.
By the way, the search approach/algorithms for the JMP documentation that is available from jmp.com (Welcome to JMP 10 Help) using the dedicated search box is different than the one used when searching within the JMP software using JMP>HELP>SEARCH. I often find things quicker when I search the docs on jmp.com. You might try that and let me know what you think.