The regular interval is used to make a macro trigger regularly after a fixed period of time. If you want the trigger to fire on a specific time boundary (such as on the hour) you should enable the option use a reference time. Otherwise it will fire at the set period of time starting from the moment the macro was saved.
It is necessary on many devices to use the alarm option if you require the trigger to fire exactly at the correct time. Without this option enabled many devices will prevent the device waking up at the exact desired time.