How to convert the date and time difference from the current date and a selected date into seconds?

Hi,

Im making a clock with an alarm, and i want to be able to set the date that the alarm will go off, however to do this i need to be able to convert the difference between the current date and the target date into seconds in order to be able to use it in the elapsed time vi. Does anyone know how to do this? Or is there a better way to achieve this?

Thanks