Ok, you'd expect that to be sufficiently solid for this box. I'd start by placing a buffer capacitor at the DC input jack; something like 470uF or 1000uF. The 100nF decoupling caps @JohnRob mentions won't hurt either, but I'm afraid they're redundant as all modules (the Nano, the RTC) usually have them on there already.
What does your relay switch, exactly?