Lets create an alarm which reminds us to take out the trash. Because taking out the trash will need to be done periodically, this situation is ideal for a periodical alarm.
We are going to look at the controls indicated by the red rectangle in the image below. First you need to select 'Periodically' from the box on the left. Now all the controls for a periodical alarm appear. This all may look very complicated, but... well... well, at least it's not VERY complicated!
Now we have to specify the period length. Taking out the trash will have to be done each week. Unfortunately, the period length can not be set in weeks. Now we remember that there are 7 days in a week, so we set the period length to 7 days.
Next, we need to choose a date and time which will act as a reference point to determine when to sound the alarm. This is the date indicated by 'include'. The trash (in the example) has to be taken out on Mondays, so we make sure the 'include' date is on a Monday. The period of 7 days will be added to this date and time. This reference point may be any date and time in the past or future. Because the alarm will sound every 7 days, the month and year you specify here don't matter, as long as the day is on a Monday. The time indicates when the alarm will sound on that day.
You see that for an alarm which sounds periodically, you can set some other options. These are restrictions to the moments the alarm will sound and are not important for the trash example. See the restrictions page for information on these options.
Note that you can select the date and time by clicking the button with the dots to the right of the date and time.
Note: if you're editing a date and time, ctrl+n will set this date to the current date and time, ctrl+c will copy the date and time, ctrl+v will paste the date and time.