You could do it with one variable, but I think you'd end up with very long and awkward expressions to cater for stuff like the varying amount of days in each month, and the way we write months and dates starting with 1st instead of 0th..
It'll be much less of a headache if you use seperate vars for Date/Month/Year.
Plus, you can create the Date string where it's needed afterwards just by using set text to Date&"/"&Month&"/"&Year
expressions you'll need are:
tokenat() I think this is what you were referring to with parsing..
int() converts a string into an integer
you'll find good explanations of each in the manual
*edit: I made a cap anyway
It's a little confusing at the bottom so if you have any questions..