can i use two capacitors each of them 68PF??
Yes.
and in this situation,is the accuracy acceptable?
remember that the circuit is working 24 hours continuously for ayear.
i am afraid of accumulating errors!!!
The frequency of a crystal oscillator depends on the load capacitance on the crystal, in this case two 22pF capacitors in series, plus the circuit stray capacitance.
2 x 22pF in series is 11pf, add 9pF for the strays and you have 20pF.
Depending on how the crystal is made, a common load capacitance is 30pF, so a 30pF crystal in your 20pF circuit would run at a higher frequency than intended.
The accuracy you will get from using twp 68pF capacitors is anyones guess, there is no way of knowing.
The crystal frequency will also change with temperature, how much it will change depends on the crystal, (and the temperature coefficient of the load capacitors).
which is more accurate : delay loops or timer zero??
They both have the same potential accuracy, however when using a delay loop you will have to calculate the number of instruction cycles to give the required delay, using the timer you just have to calculate the division ratio depending on the clock frequency 4Mhz in this case.
If you want long term accuracy, it would be better to extract the timing from a GPS module or a radio controlled clock module, then the time will be accurate to 1 second in a hundred years or so!.
JimB