I am sure this happens with lawyers routinely. Usually a deadline will be expressed in number of days from some event, like “90 days from the date of agreement”

I am sure this happens with lawyers routinely. Usually a deadline will be expressed in number of days from some event, like “90 days from the date of agreement” or something. If it is an important deadline, do not trust your lawyer to do this day count correctly. Ensure all of the assumptions are correct (date and time of events, whether it is business days or calendar days, etc) and triple check the day counts on a calendar yourself.