You may want to base your decision on who your prospective investors are. Professional investors, including members of angel groups and VCs, are used to seeing Y1...Y5 and are realistic enough to know that you don't actually know when your funding is going to come in. But for less sophisticated investors who may not be used to thinking that way the calendar years are probably best.
Even for professional investors the use of actual years shows confidence and determination, but if you are at all delayed you will find yourself constantly updating your projections.
Just as you have to for your business product or service, think about who your target market is and then make a decision.