Do not save dates as timestamps

For some reasons it is a relatively common practice to save dates as timestamps, but it should be avoided at all cost.

The following scenario illustrates the problem

A web page lets a user choose a day when he expects delivery of a product. He chooses 2017-09-15 (September 15th). The web page then sends the date to a web service. The web service expects a timestamp with millisecond resolution. To achieve this the JavaScript developer of the page had a number of options where all of them were bad. Let us say they she chose to take the millisecond at midnight where the day ends in the users' time zone. So the millisecond is now 2017-09-14T15:00:00.000Z since the user of the web page lives in Yakutsk. Two days later the user calls a call center in New York and says that he wants to move the delivery date to 2017-09-14. The call center representative gets a date picker for 2017-09-14T15:00:00.000Z and since he is in New York that time is actually 2017-09-14T10:00:00.000 in local time. So that date picker displays 2017-09-14, which is already the date that the user wants to change to. The representative gets confused, tells the user that that is already his delivery date and does nothing more.