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To confirm, would you like your users to only enter future dates within your Date and Time fields?
If that is the case, you have the option to disable past dates. As such, they would not be able to select and previous dates.
Please note that this settings only applies to the date, users will be able to select previous hours on the current date.
However if you set it to default time and change the minute step up to 30 minutes, when the form loads it would load with the closest 30 minute interval to the current time.
Please inform us if you need further assistance.