timestamp in custom made column in outlook 2013

good day i need a script that will add on a custom column in outlook 2013 a timestamp whenever a specific carather flag is added on the follow up flag column or a category is added on the category column (either will do) we do work with shared email and i need to measure when a mail is flagged from user A (which has to qualify the mail ) compared to when the email is received so that i can monitor the lead time since mail receipt. subsequently, user B will reply to the mail separately so i cannot use internal outlook timestamp but need to add it the moment user A has qualified the email. possibly the time stamp should not be possible to be modified so that user A cannot tamper with data.