When you set a date (or a Next Step), Sortd will automatically add a reminder for that date (set to notify you at 7am by default).  If, for example, you have set a next step and you want to change the reminder, you would click to open the card in detail view.  Here's what the detailed view looks like on the Sales board for example (the dates and reminders work the same on all the boards though):

To change the reminder date, click on it and select a different date / time, or to remove it completely, just click on the 'x'.