It appears there is a bug in the "Data Usage" feature of Avast Mobile Security (Pro) for Android.
My data usage cycle runs from 00:00 on the 21st, to 24:00 on the 20th.
Today, it is telling me I've used 138 MB this cycle, which happens to actually be the total of what was used on the 20th plus today's usage.
If I change the setting of "Day of month to reset" from "21" to "20", it then reads 164 MB, which is the total usage of the 19th through today.
If I change the setting of "Day of month to reset" from "21" to "22", it reads 1.16 GB, which is the total usage of the entire last cycle, plus today's usage.
This is clearly an off-by-one error in the program logic.
Who do I contact to report this?