Alarm clock for Ubuntu

These 3 should all do what you require and are in the repositories:

https://apps.ubuntu.com/cat/applications/precise/alarm-clock-applet/

Alarm Clock is a fully-featured alarm clock which resides in the notification area. It is easy to use yet powerful with support for multiple and repeatable alarms, as well as snoozing and a flexible notification system.

Two types of alarms are supported: Alarm Clocks and Timers. Notification is done by either playing a sound or launching an application.

https://apps.ubuntu.com/cat/applications/wakeup/

This package has a complete graphical front end with which a user can set an alarm to wake the computer - from poweroff if possible - and read a user-defined text. This text can grab relevant information (date, time, weather, Evolution schedule and tasks, news from an rss feed, number of new email messages, etc.) and speak that as well, or play music, all as defined by the user. More capabilities can be added to the alarm via a complete and simple plugin system. Supports multiple alarms.

https://apps.ubuntu.com/cat/applications/alarm-clock/

Alarm Clock is the personal alarm clock for GTK+ desktop environments. It supports sound fading, scheduled alarms, snooze option, passive window reminders, exception lists for scheduled alarms, exporting alarms and much more!

By Terminal, you can use this command:

sudo apt-get install alarm-clock-applet

Source: http://www.ubuntubuzz.com/2015/05/how-to-install-alarm-clock-in-ubuntu-1504.html

You might want to have a look at Teatime: