//endDate is 31.10.2018
def format = messageService.getMessage(code: 'default.date.short.format')

//--> dd.MM.yyyy for DE and MM/dd/yy for EN
println("Use format: " + format)

SimpleDateFormat sdf = new SimpleDateFormat(format);

//set timezone (America/New_York)
sdf.setTimeZone(TimeZone.getTimeZone(user.timeZoneID))

//parse endDate
Date parsedEndDate = sdf.parse(endDate)

//create a calendar instance (e.g. America/New_York)
Calendar calendarEnd = Calendar.getInstance(TimeZone.getTimeZone(user.timeZoneID));

//set time
calendarEnd.setTime(parsedEndDate);

//set hour/minute automatically
calendarEnd.set(Calendar.HOUR_OF_DAY, 23)
calendarEnd.set(Calendar.MINUTE, 59)

//at this point it should be 31.10.2018, 23:59 (german format, timezone America/New_York)

//Convert to UTC before saving date in DB (MySQL)
calendarEnd.setTimeZone(TimeZone.getTimeZone('UTC'))

//save the date
def obj = new Foo(date:calendarEnd).save(flush:true)

<g:formatDate
        timeZone="${user.timeZoneID}"
        date="${fooInstance?.calendarEnd}"
        format="${message(code: 'default.date.format', default: 'MM/dd/yyyy, hh:mm a')}"/>