Alarm Clock
This is a complex window, allowing you to schedule your future appointments. To be notified of any of these, you need to have this application running. It can be running the background, and will sound and then show an alarm alert. If you are working in another application, you can read the alarm by clicking on its window or using the application menu at top right.

Setting an alarm
To set an alarm, you must type in a message to appear, and then set the time at which it appears using the popup menus for the date, month, year and time. You are limited to 5 minute intervals. To set this alarm just click the Set button (or press the Return or Enter key). If you want to be pre-warned before this date and time, then you can use the Prior warning popups. To set the alarm to recur once you have read it, set the time for the recurring alarm using the Repeat: popups. The top one determines the mode, which can be using a specified time, by a specific day of the week, every weekday or never.
When you click Set, your message and time due will appear below in the Pending Alarms list. To review a one of these, just click in the window on it. The alarm you want will be selected and will reappear with its dialog settings. You can then alter it and click Set to resave it. You will be asked if you wish to replace the prior alarm. If you do, it will be deleted and your new alarm set in its correct place, otherwise the old version will remain. Note that the alarms are filed in due order (which depends on the pre-alarm time not necessarily the date you wish to remember).

Responding to Alarms
A dialog will appear when an alarm is due. This is a modeless dialog, which means you do not have to respond to it at once. You can manipulate any other windows in this application, or even leave it entirely until you are ready. All other alarms will be suspended until you enact this box. In addition, you can even delete alarms and this will not affect the performance of this dialog. The dialog will allow you either:
(1) to Postpone the alarm (the default time is 5 minutes, but you can set another time for the next reminder). There are two options for setting a reminder time. You can postpone by a specific interval, or to a specific time/date. The choice is determined by the radio buttons, and their associated pop up menus. Look to the bottom text field for the actual time to which you will be setting the postponed alarm. Note that you can set weird times with these popups, but the actual time will be shown at bottom; or
(2) Dispose It (presumably because you have taken appropriate action). Note that if the alarm has a recurring time set, then this will not be enacted until you use Dispose It on the current alarm. The time set will of course be set from the original time you set, eg if you want to be reminded annually for a birthday, then the date will be correctly set, and the prewarning will also be recalled. If you want to completely dispose an alarm, whilst setting a correct future occurrence of that alarm, then click the Keep New button at bottom left (this will delete all current reminders for this alarm too). This is useful if you have been away from your Mac for a while, eg on holidays, and many alarms have accumulated. You will only need to click this button once for each alarm to set them to the correct future time.

Other Options
If you have selected a Pending alarm, and find you cannot de-select it, then click the New button. This avoids being asked if you want to replace the selected alarm each time. The New button will not alter the message in the top field, so you may make small changes to set multiple similar alarms. To display this help click on the Alarm clock icon. If you select 12.00am or 12.00pm then you will note the day is followed by either midnight or noon (since I find these confusing and wish to be reminded!).