//Create date variable
Date theDate = currentValues["Date Field"] as Date
//Create calendar variable for date manipulation
Calendar calendar = Calendar.getInstance()
//Set calendar date to date variable
calendar(theDate)
//Add one month to date
calendar.add(Calendar.MONTH, 1)
//Set calendar date to the first day of the month
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMinimum(Calendar.DAY_OF_MONTH))
//Set date variable equal to the calendar variable
theDate = calendar.getTime()
//Set date field to the new date
currentValues["Date Field"] = theDate