Need to make a text box "Flash" in a web app

I have a web page where users enter dates into fields.  There is a note on the screen that says to please enter the dates in mm/dd/yyyy format.  When they enter a date in another format, I overlay that note with an error message in red text on a white background, but apparently some users still do not see it.

What code can I use to make that error message, white background and all, "flash" until they correct the date?

Where do I place the code?