You should be able to find the total uptime for the month on the computer webpage itself, but there's no easy way to include this information in a SQL report. What is stored in the database are just the uptime events: reboots, shutdowns etc. Computer webpages use a special procedure to convert this information into an uptime calendar.
We did post a sample report that summarizes uptime information
here, but this will only work for databases hosted in SQL Server. The report can be slow to run as well and is not easily modifiable.