Bukkit

If it is possible, This plugin could calculate monthly uptime percentage. (or any predefined date if one wanted).

To avoid server crashes (or whatever downtime) changing data, the plugin should lay down a marker every few minutes (ex 5 minutes).
As the server date changes, previous days data would be calculated and missing markers will be considered as downtime and compressed into 1 data point per day (daily percentage) and day markers would be whiped and added to month list.

So pretty much this should go down.

create day.log
9-11-12 (logged day date)
00.05
00.10
00.15
01.45
01.50
01.55
...

And so on.

I added in a 1.5h downtime (no checks in file in between 00.15 and 01.45)
If the day ends (date changes). All data from day.log will be put into month.log
09-11-12/18/91.96 (date/missed checks/day uptime percentage)
10-11-12/...

So pretty much what I mean that after certain time period the file would always be compressed smaller (old data whiped (calculations made))
Putting out a percentage

This is my idea, and could fit together with yours!
Happy coding! (if you accept my enchantment)