There *is* a way... it's not perfect, and it works best for once a day, or random updating.
If you have a cgi-program that requires parameters, or to be called from a <POST> command, a certain number of calls a day will not have the required data. Either browsers mess up, people don't fill in all fields, or someone tries to hack the program (innocently or maliciously).
If you scan your input for missing, erroneous, or otherwise 'invalid' data, and every time you find it, you call your "pseudo-crontab program" to run, you have effectively (albeit haphazardly) accomplished your goal.
If you only want the program to run ONCE per day, or per unit time, write a small "Updted_already" file, and put in the date and/or time. If that date contained in that file is newer than the required update cycle, ignore it, otherwise, rewrite the file, and do your thing.
Poor man's crontab