How can I ensure in Java that the current local time is correct? [duplicate]

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Radio Clock

Obtain a radio clock with a USB connection for output of current time synchronized by a time code transmitted by a radio transmitter connected to a time standard such as an atomic clock. Similarly, a receiver of GPS signals could also capture the time signal.

The Meinberg Global company, at least, offers several such devices.

photo of a couple of radio clock via USB devices

Sundial

Position a sundial outside a window. Attach a webcam to the computer in question. Position the webcam in the window. Write an app to interpret the time of day from current image of the sundial. Caveat: Not applicable in Seattle.

photo of a sundial on a pedestal in a garden

Black Box, No Inputs = No Go

More seriously, if you do not trust the system time, you have no access to a time server, and you cannot ask the user for current time, I cannot imagine any solution. You have a black box with no inputs. The radio clock via USB is adding such an input.

Wikipedia has an article on Clock synchronization.

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