Use the p5 text injection datetime method 
with usb upload 
(add a line of text into the uf2 that has the datetime somehow - would need to figure out how to make it work with uf2)

then, on the rp2040 side, set the time to that and then wait say 5 minutes (the person has 5 minutes to plug it in) 

then if the datetime is not 0 and is within say 15 minutes of that hardcoded time, turn off the set time flag? not sure if that works

maybe if its more than 5 minutes off but less than an hour off, turn off the flag that sets the time and the flag that checks if the time is in the right range? 

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using low frequency waves from headphone jack 
ie 5hz and 7hz to send a binary number thats the epoch

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just having a switch that turns it into set mode and uses serial communication to set the clock

ie
https://editor.p5js.org/jps723/sketches/Byvw1lu6Z

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could do both so that a software update automatically sets clock 

or maybe just having both be one thing really is the best way