Design pattern to modify an epoch timestamp on modbus tcp

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Hi,
I've a modbus slave talking to mango 2.8.8 , I want to show on the web interface an epoch coming from the slave, I've done this using the datapoint rendering

but this is not a clean solution because:

1. I can't show the right time including locale without showing the locale itself
2. to modify the value on the slave I've to send to it an epoch and not a formatted date

What is the cleanest way to obtain this behaviour (modify an epoch on a modbus slave)?
Is there some example for this?
Thanks,
Antonio

Using a Meta Data Source I think could be possible to format the date as I need but how I can then map to an epoch as required when the user for example change the hour?