At every interrupt this is what happens in the INT routine.......so actually the number of instructions to be executed, change. So I am not sure tweaking the timer pre-load value will help.
Code:
sec=sec+1
			if sec =60 then
				sec=0
				minu=minu+1
					if minu=60 then
						minu=0
						hrs=hrs+1
							if hrs=24 then
								hrs=0
								days=days+1
									if days=8 then
										days=1
									endif
							endif
					endif
			endif