can I set the date for an alarm with the alarm extension from pura vida? and if that so, how can I do it?
No you can't
See also Q4 from the FAQ here App Inventor Extensions: Alarm | Pura Vida Apps
Unfortunately the AlarmClock API does not offer these features, see also here. But you can set the weekday for repeating alarms in the paid version of the extension. You will need min. API Level 19 (Android 4.4) for that.
Taifun