What does one need to deal with or know when working with dates and times?

Around the internet I've found a lot of little tricks that slip up programmers when working with dates and times.

Some examples include:

• Not all timezones are whole numbers
• The 100/400 rule with regards to leap years
• dd/mm/yyyy vs mm/dd/yyyy
• The 2038 problem
• etc

So what should a programmer dealing with dates and times know? Please keep each piece开发者_Go百科 of information in its own answer.

What issues are involved when dealing with timezones? Differing days around the world. Etc etc etc.

I am not looking for trite answers "using the existing" functions because that answers precisely nothing. It does not tell me what issues are involved when writing a time heavy application (say a calendar)