I went to get a DS1307 RTC and the required 32.768kHz 12.5pF crystal.
They didn’t have a 1307, so I took a 1302 instead… I know the interfacing is different and have found a library for it.
BUT… only when I got home did I realise that the 1302 requires a 32.768kHz crystal of 6pF, but I had purchased one of 12.5pF.
Is that likely to be a problem? (Confession: I don’t even know what it means for a crystal to have a capacitance, although I do know what a capacitor is… 8) )