You already solved your issue, but still I'll mention it:
For my Pi's I buy a separate RTC (uses GPIO pins). There's various available, the DS3231 is the most expensive but also the best. If you buy it in bulk (70+) I suppose you could shave some money off. If you're OK with a cheaper one (less than half the price) which functions less well (but still does the job) then PCF8523. I got mine from a local Adafruit reseller.
FWIW, the cheapest RTC module we could find that had kernel support was about $31 at the time. Things have gotten a lot better and cheaper since then. Adafruit didn't launch that DS3231 until four years after we did this.
We couldn't justify adding ~$2500 to the bottom line, but you can be confident that we sure did argue about it.
Missed this comment when asking why you didn't add a $2 RTC, but given that you mention Adafruit I assume making your own hardware to stick onto a Pi wasn't an option? The DS3231 you mention is a chip with mainline support since 2009. Cheaper options like a DS1307 cost less than a $0.50 a piece when you buy 100 of them, and they've been supported in Linux since 2006.
I'd say 50/50 that I didn't know what I2C was in 2014.
Still, if we had gone and designed our own RTC daughterboard, it would have replaced the UBEC in our architecture diagram. We were buying those for $3-4 in 1000 unit quantities.
So, had you been on hand a decade ago, you might have won the argument although if I put on my board member hat, fabbing our own PCB sounds a lot scarier than buying boxes of buck converters.
It's hard to express how wild it is that they just pop right on and guarantee close to perfect voltage to each device.
Fair enough! I'm not sure I caught that the project was from 2014.
From your URL, the second part made me wonder if the statement is correct: "UBEC 5V 3A/5A/7A/15A BEC 2-12S Lipo Step-down Module External Power Supply Full Shielding Antijamming For FPV Airplane" would this actually work?
For my Pi's I buy a separate RTC (uses GPIO pins). There's various available, the DS3231 is the most expensive but also the best. If you buy it in bulk (70+) I suppose you could shave some money off. If you're OK with a cheaper one (less than half the price) which functions less well (but still does the job) then PCF8523. I got mine from a local Adafruit reseller.