Spain - VISA exemption policy

Dear Sir/Madam,

I am Spanish and I am travelling from Zurich to Hanoi via lay-over in Hongkong. Leaving Switzerland on the 2-dec and Arriving in Hanoi the 3-dec.
I will leave from Hanoi to Zurich via lay-over in Hongkong the 17th dec arriving to Zurich the 18th dec.

My doubt regards the way of counting the days (24h periods or solar days, from start to end of trip or from arrival-departure to/from Vietnam) and the fact that I see both (exempted for VISA when travelling less than 15 days and need a VISA when more than 15 days). My trip being around 15 days, I'm a little confused.

Could you please let me know if I need to apply for a VISA for the above described trip ?

Thanks & Regards

Aitor