You have a timer and clock in just about every single device out there these days, but if you want to build you own, Instructables user GeraldF6 shows you how to do so with a Arduino.
The project here is a smart voice controlled clock that does not require an internet connection. On a basic level, this means you can ask it what time it is, what day it is, or set a timer. Beyond that though, you can expand that programming to include more options and with a little tinkering you can likely get a display on here as well, making it a good candidate for a DIY alarm clock or something similar. As it stands right now, it's pretty simplistic, but it's a good starting point for a much more complex project. Head over to Instructables for a full guide.
Voice-Controlled Clock with Arduino (No Wi-Fi, No PC) [Instructables]