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.  

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