Even big names such as Apple, Microsoft, Twitter have had problems with time causing bugs that meant that APIs, Zunes or iPods would stop working. Many of these bugs could have been prevented with better general knowledge about time.Most programmers have to deal with time issues such as DST and timezones. But few know the difference between GMT, UT1, UTC and TAI.The talk first provides a short introduction to how modern time measurement evolved and how it works today. Covering solar clocks, trains, caesium atoms, astronomy, timezones, legislation, leap seconds and more.A particularly interesting part of the presentation is a Phoenix app showing a live clock with the relationship between the Daylight Saving Time, atomic time, timezones and leap seconds.Tzdata is the only Elixir library for providing timezone information. I will explain how the first versions relied heavily on macros/metaprogramming. And why I changed it so the new version uses ETS tables and makes more use of OTP concepts and concurrency. This use of OTP means you can have more up to date timezone information on Elixir than on other platforms.Then, some general recommendations and best practices for developing software that has to work with date and time.Finally an overview of Calendar (and related packages for Phoenix and Ecto) and how they incorporate the aforementioned best practices.