How accurate the an attiny timer function I have an AT-Tiny (trinket) and I want to trigger LED on for 30 seconds every 24 hours. I'm using the timer library as follows 

timer.setInterval(86400000, toggleit());

void toggleit()
{
  turnLightOn();
  delay(6000);
  turnLightOff();               
}


How accurate will this be, if it's off by a few minutes each day that's fine, but will this drift over time? Also does it make a difference if I'm running it in 8 or 16 bit mode?
The on-chip systemclock is not very accurate. You can calibrate it to some degree but it also depends on the temperature and load of the MCU. You will need a high quality quartz oscillator as external clock source to reduce it. But it will still have a drift. Switching from 8-bit to 16-bit won't make a difference.

In your case an RTC sounds like the way to go.