How are your OS system settings set? You could change your system setting to the format you want and tell FM to use the system settings.
quoting a FM doc ...
The Date ( month ; day ; year ) function has three parameters. FileMaker Pro always separates parameters with
a semicolon: Date ( 12; 25; 2015 ) will return "12/25/2015" as a date.
Your solution may format results differently based on your date settings. However, the Date function shown above will have the same order of parameters in any solution
Are you accessing via Webdirect? If so there are some more complicated instructions for changing that.