@DavideB. I sympathize with your frustration. The thing to remember is that we are on the bleeding edge of development and we can't (should not?) expect things to "just work". Maybe next year it will be Plug-n-play
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From your description, I understand tht you ran your CC ENVI monitor for one week not attached to a PC. (And, I assume, not attached to a CC Bridge). This means you were relying on the ENVI to do your data logging. The ENVI has two sets of outputs "Real time" and "History" described in . The real-time output is not saved so for your data logging and thus subsequent feed to GPM you can only use the history output which comprises last 31 days of 2-hourly totals (not averages), last 90 days of daily totals and last 7 years of monthly totals. So 2-hourly totals is the best precision you can achieve.
You say that 2-hourly totals is "invaluable" and most of us using the CC ENVI would agree that 2 hourly is useless for some of the things we want to do. {BTW, by a quirk of the English language invaluable means valuable in the sense of "unable to calculate the value"}. This means we need to do the datalogging external to the ENVI (using PC or CC Bridge). In my case I am logging both Energy Usage, and, Energy generation from a solar Photo-Voltac System. That means leaving the ENVI connected to an always-on PC (laptop) and logging the real-time data. The real-time (6 sec instantaneous Power values) is rolled up to 1 min Energy and written to files for subsequent analysis. I've been doing this for about 1 year using Techtoniq Energy Station, a low-cost commercial program. I also publish to Google power Meter using the Current Cost Power Meter Interface.
This year I have also been publishing to PVOutput.org using their provided software which supports CC ENVI for both power generation and power usage. This is a free service and has vastly superior graphing and comparison facilities. See/explore .
Getting back to your problem of missing 4 days. The only explanation I can think of is that you may have adjusted the clock? It's quite easy to add a day or two but 4 does seem excessive. If you've only recently purchased your ENVI and you know the date it was first turned on you could check the <DSB> node (days since Birth) in the XML output is still correct.
Good Luck HTH.