Based on XDSoft DateTimePicker documentation, here's what can be done:
var logic = function( currentDateTime ){
var d1 = new Date();
// Check that it's today, so we need to restrict time chooser
if (currentDateTime.getDate() == d1.getDate() && currentDateTime.getMonth() == d1.getMonth())
{
// Adding six hours
d1.setHours ( d1.getHours() + 6 );
// Creating 'HH:MM' string
var defaultTime = (d1.getHours() < 10 ? "0" : "") + d1.getHours() + ":" + (d1.getMinutes() < 10 ? "0" : "") + d1.getMinutes();
// Enforce time restriction
// ('this' is jquery datetimepicker object)
this.setOptions({
minTime : defaultTime,
defaultTime : defaultTime
});
}
else
{
// Lift time restriction if selected day is not today
this.setOptions({
minTime : false,
defaultTime : false
});
}
};
// Initiate datepicker with custom logic
$('#datetimepicker').datetimepicker({
onChangeDateTime:logic,
onShow:logic
});
Adding 6 hours solution based on: https://stackoverflow.com/a/13034220/2715393