How do I get the uptime percentage for a monitor?

Yes, and that is the error you hit. Both the start and the end of the window are mandatory for the timeline and for the response time chart, and the end must be later than the start. There is no default window. For a monthly report: 1. Set the window from the first day of the month to the first day of the next month. 2. Read the uptime percentage from the summary of the response. It is computed as up seconds divided by the total seconds of the window you asked for. 3. If you also want latency for the same report, use the response time chart with the same window. It buckets the data for you, and if you do not choose a bucket size it picks one from the window length: five minute buckets up to a day, hourly up to a week, six hourly up to a month, daily beyond that. One caveat worth stating in your report: the percentage is derived from the checks that actually ran in that window. If the monitor was inactive for part of the month, those hours simply have no data rather than counting as down.