If you don't care about many details and you just want the up and down time. Then you could save 2 numbers in a file, one being the current percentage and the other being the current number of measures the script took.
For example: '96 800'.
Then, whenever the script is executed, use this formula, assuming that the percentage (96) is X, the number of saves is Y (800) and the current measure is Z (0 or 1)
X=X/100;
X=(X*Y+Z)/(Y+1); //You get the new value of the percentage, X. Be careful as they need to be long values.
Y=Y+1; //And the current number of interactions.
X=X*100;
That's of course just pseudo code and won't work straight away, it's just to give you an idea of one way to do it. You must also include a way of reading a file and saving it wherever you want of your page that contains those 2 numbers.
As Jamie stated, this is assuming that crontime is still working despite of the server is down.
PS, you could also add a system from other server to check this server uptime, but it would need to be statistically calculated (and error tends to 0 when many other servers are added, but that'd be too difficult) as you'd need to take into account the other server's uptime also...