If statement in isr

[Ttelmah]

Crystals require two capacitors not one.
33pF, sounds rather large. Depends on the specification for the crystal you
use and the capacitances of your board layout in this area.
A typical 'watch' crystal, will be rated for a load capacitance of perhaps 12pF
to 20pF.
Now the external capacitors need when added to the track and chip
capacitance and treated as being in series across the crystal, to give it's
rated capacitance. The typical chip input gives a capacitance of perhaps 4pF.
Your traces typically if kept short about the same. So the trace and pin
give between perhaps 7 and 10pF. Some watch crystals are designed for
a load capacitance of only about 7 or 8pF, and these are designed for use
without any external capacitances. For a 20pF rated crystal, the capacitors
needed would be:

(20 * 2) - 9 = 31pF (assuming track & pin=9pF)

So this would be the crystal that wants about a 32pF capacitor.

However for the 12pF crystal, the capacitance would want to be:

(12*2) - 9 = 15pF

This would be a much more typical value for a 'watch' crystal. Most of these
are rated for capacitance's like this.
The '*2', is because of the capacitances being treated as being in series.

[temtronic]

As Mr. T points out there is some PROPER design of PCB and components that needs to be done to get a 'watch crystal' running correctly.
Be sure to clean the solder joints and PCB. Any residue left can affect timing, especially in humid condiftions.
Providing you're using a real crystal for the main clock, the SW RTC code, should/would be more accurate over the long haul.
Also, if a long term project, you'll need battery backup and SW to keep the clock ticking !