The LPC21xx real time clock relies on dividing the system clock down to 32,768Hz. This is its greatest weakness, since in Power Down Mode, the clock will stop ticking.
It would have been nice of Philips had given us an external 32,768Hz clock input so we could stick on an external oscillator to keep things running in power down mode, but it looks like the standard answer is going to be to attack an I2C external real time clock and use that to initialise the internal RTC after every reset / wakeup.
Having said that, it's a nice RTC - you can set alarm interrupts and everything!
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