No, sounds as if you are trying to turn the time back too far. Wait until later in the day and try again. How much are you trying to adjust it by? If it is trying to go back a full day, because of your locale, you may need to reset the user data. Basically factory reset. Then set the time. That way none of your data is there to confuse the stoopid machine. Once that is done, then you can reset it to your treatment pressures. The card will have your data which you should copy over to your computer or, if you use SleepyHead, just upload the latest. Unless you told it otherwise, SH does a backup of the card data. When you put the card back in the machine, it may want to format it.
But keep in mind that if you need to demonstrate compliance, most will use the card or the machine data to "prove" it. Doing this will erase that proof from the machine and probably the card. You could get another card and keep the first one in this case.
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