Cannot verify account because of system date error

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I've just upgraded a client to Timeslips Premium.  At the end of the install process, the installer tried to verify the registration but it keeps failing with the message, "Sage Timeslips Premium cannot verify your account.  Your system date does not match the current date."  Of course, I have verified that the current system date, time and timezone are correct.  The software claims it will stop functioning in two days if this isn't resolved.  Does anyone know what the problem could be?