There is CISS register at lpc23x/24xx, but I can't find it at lpc17xx_v07 user manual
Chapter 27: LPC17xx Real-Time Clock (RTC) and backup registers states that Periodic interrupts can be generated from increments of any field of the time registers and selected fractional second values.
But where is those selected fractional second values ?
It's an NXP chip - so try the NXP website first?