My SX100 drains 0.00 ampere when its on my power supply, without batteries. I guess the RTC takes its power from the button battery alone.
I remember the battery of my A410 was perfectly fine for 4 years. (Camera dead by now, battery still fine, runs my grandmas wristwatch now
)
Also, i remember the clock always being extremely accurate with little to no drift..
Out of curiosity: Why do you need such extreme accuracy? Would it be really that bad if a picture is 1 sec off?