APIs dealing with dates and times must be hard to get right. I'm not aware of any programming language whose standard library for dealing with times is both intuitive and comprehensive. Racket has built-in libraries that are second to none, but even it presently leaves something to be desired when it comes to support for handling dates and times.
For example, Racket's racket/date module (in its current incarnation) includes a date->string function, but no string->date function.