truncate(Date date) method returns the date rounded to the day with a time of 00:00:00:00. setEndOfDay(Date date) method returns the date rounded to the day with a time of 23:59:59:999.

public DateRange getDateRange(Date fromDate, Date toDate){
	if (fromDate!=null && toDate!=null )
	{
		fromDate = DateUtil.truncate(fromDate);
		toDate = DateUtil.setEndOfDay(toDate);
		range = new DateRange(fromDate, toDate);
	}
		
	return range;
}

getDateDiff( int calculationUnit, Date d1, Date d2 ) method returns the difference of the two dates in the given calculation unit. 

public Boolean expired(Date fromDate, Date toDate, Integer daysValid){
	
	Integer numberOfDays = DateUtil.getDateDiff(Calendar.DAY_OF_YEAR, fromDate, toDate);
	
	if(numberOfDays>daysValid)
	{
		return true;
	}
		
	return false;
}

 add(Date date, int timeValue, UnitOfTime uot) method adds the given number(time value) of units to the given date.

public Boolean getCompletionDate(Date startDate, Integer timeValue, UnitOfTime uot){
	
	Date completionDate = DateUtil.add(startDate, timeValue, uot);
		
	return completionDate ;
}

Alternatively, any of the following methods can be used if the unit of time is known and not given. 

addYears(Date date, int numberOfYears) method adds the given number of years to the given date.

addMonths(Date date, int numberOfMonths) method adds the given number of months to the given date.

addWeeks(Date date, int numberOfWeeks) method adds the given number of weeks to the given date.

addDays(Date date, int numberOfDays) method adds the given number of days to the given date.

addHours(Date date, int numberOfHours) method adds the given number of hours to the given date.

addMinutes(Date date, int numberOfMinutes) method adds the given number of minutesto the given date.

addSeconds(Date date, int numberOfSeconds) method adds the given number of seconds to the given date.