That's not a problem... learning opportunuities.

USE I2CWRITE with the same settings as I2CREAD should work

you just need to test your pushbbuton...
Code:
lcdout $fe,1 ' Obrisi displej 
lcdout $fe,2 ' Prvi red prvi karakter
lcdout "Time: ", hex(H),":", hex(M)," ", hex(S)
lcdout $fe,$c0 ' Drugi red, prvi karakter 
lcdout "Date: ", hex(D),".", hex(Mn),".20",hex(Y)
                    '<== Here
Pause 500     
goto Main ' Ponovi sve
but certainely better if you use something like
Code:
lcdout $fe,1 ' Obrisi displej 
lcdout $fe,2 ' Prvi red prvi karakter
lcdout "Time: ", hex(H),":", hex(M)," ", hex(S)
lcdout $fe,$c0 ' Drugi red, prvi karakter 
lcdout "Date: ", hex(D),".", hex(Mn),".20",hex(Y)

While Delay<5000 ' Delay loop of 500 mSec
     ' Select case pushbutton and set/hour time here+I2CWRITE
     ' if there's no push button pressed Do a PAUSEUS 100 and increment
     ' Delay Variable.
     '
     ' 
     Wend
goto Main ' Ponovi sve
yeah just an idea. not everything on a plate and you'll have to do some homework around the above.