If you want to use your webmail as a calendar, Horde is the only webmail client currently offered that comes with a built-in calendar that you can use to send reminders to yourself or any other contacts in your Address Book. 

To access the Calendar, first log into your webmail and select the Horde webmail client.  Once you have logged in, select the Calendar button on the top menu to load the calendar:

There are two ways to add events and reminders to your calendar.  You can click the + icon next to the date to add a new event, or you can click the New Event button on the Menu.  Once you have chosen by either method to create a new event, you will be able to put in the event information:

Horde Calendar is equipped to handle a range of options for your new event, including the time, description, where the event is, if it is recurring, and much  more.  In the example above we have created a simple event to remind us of a team meetings.  Once the information is filled in, click on the Save Event button.  Now the event will show in your calendar:

You can edit your event at any time by clicking on it on the calendar, and you can add as many events as you need. For a demonstration, please see the video provided.