//Change timezone if needed 
def tz = 'IST'  
TimeZone.setDefault(TimeZone.getTimeZone(tz))
 
def dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
def dateString = "2017-12-18T20:41:06.136Z"
 
Calendar calendar = Calendar.getInstance()
calendar.with {
  time = Date.parse(dateFormat,dateString)
  set(Calendar.HOUR_OF_DAY, 0)
  set(Calendar.MINUTE, 0)
  set(Calendar.SECOND, 0)     
  set(Calendar.MILLISECOND, 0)
}
println calendar.time.format(dateFormat)

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