How to get the start and end times of a day

In my C# app, I pass a string variable that is of format yyyymmdd-yyyymmdd that represents a from and to date. I want to get the start and end times for these dates respectively. Currently I have the below code but was wondering if there was more of an elegant solution?

So for pdr = 20090521-20090523 would get "20090521 00:00:00" and "20090523 23:59:59"

private void ValidateDatePeriod(string pdr, out DateTime startDate, 
                                out DateTime endDate)
{
    string[] dates = pdr.Split('-');

    if (dates.Length != 2)
    {
        throw new Exception("Date period is of incorrect format");
    }

    if (dates[0].Length != 8 || dates[1].Length != 8)
    {
        throw new Exception("Split date periods are of incorrect format");
    }

    startDate = DateTime.ParseExact(dates[0] + " 00:00:00", 
        "yyyyMMdd HH:mm:ss", null);
    endDate = DateTime.ParseExact(dates[1] + "23:59:59", 
        "yyyyMMdd HH::mm:ss", null);
}

I am surprised to see how an incorrect answer received so many upvotes:

The correct version would be as follows:

public static DateTime StartOfDay(this DateTime theDate)
{
    return theDate.Date;
}

public static DateTime EndOfDay(this DateTime theDate)
{
    return theDate.Date.AddDays(1).AddTicks(-1);
}

You could define two extension methods somewhere, in a utility class like so :

public static DateTime EndOfDay(this DateTime date)
{
    return new DateTime(date.Year, date.Month, date.Day, 23, 59, 59, 999);
}

public static DateTime StartOfDay(this DateTime date)
{
    return new DateTime(date.Year, date.Month, date.Day, 0, 0, 0, 0);
}

And then use them in code like so :

public DoSomething()
{
    DateTime endOfThisDay = DateTime.Now.EndOfDay();
}

If you are only worried about .Net precision...

startDate = DateTime.ParseExact(dates[0], "yyyyMMdd");
endDate = DateTime.ParseExact(dates[1], "yyyyMMdd").AddTicks(-1).AddDays(1);

You really don't need to concatenate extra values onto the string for the time portion.

---

As an addendum, if you are using this for a query against, for example, a database...

startDate = DateTime.ParseExact(dates[0], "yyyyMMdd");
endDate = DateTime.ParseExact(dates[1], "yyyyMMdd").AddDays(1);

With a query of...

WHERE "startDate" >= @startDate AND "endDate" < @endDate

Then the precision issues noted in the comments won't really matter. The endDate in this case would not be part of the range, but the outside boundary.

public static class DateTimeExtension {        
    public static DateTime StartOfTheDay(this DateTime d) => new DateTime(d.Year, d.Month, d.Day, 0, 0,0);
    public static DateTime EndOfTheDay(this DateTime d) => new DateTime(d.Year, d.Month, d.Day, 23, 59,59);
}

The DateTime object has a property called Date which will return just the date portion. (The time portion is defaulted to 12:00 am).

I would recommend as a more elegant solution (IMHO) that if you want to allow any datetime on the last day, then you add 1 day to the date, and compare to allow times greater than or equal to the start date, but strictly less than the end date (plus 1 day).

// Calling code.  beginDateTime and endDateTime are already set.
// beginDateTime and endDateTime are inclusive.
// targetDateTime is the date you want to check.
beginDateTime = beginDateTime.Date;
endDateTime = endDateTime.Date.AddDays(1);

if ( beginDateTime <= targetDateTime &&
     targetDateTime < endDateTime )
   // Do something.

I use the following in C#

public static DateTime GetStartOfDay(DateTime dateTime)
{
    return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day, 0, 0, 0, 0);
}
public static DateTime GetEndOfDay(DateTime dateTime)
{
    return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day, 23, 59, 59, 999);
}

Then in MS SQL I do the following:

if datepart(ms, @dateEnd) = 0
   set @dateEnd = dateadd(ms, -3, @dateEnd)

This will result in MS SQL time of 23:59:59.997 which is the max time before becoming the next day.

You could simply use:

new DateTime(dateTime.Year, dateTime.Month, dateTime.Day, 23, 59, 59, 999);

Which will work in MS SQL, but this is not as accurate in .Net side.

That's pretty much what I would do, with some small tweaks (really no big deal, just nitpicking):

• The TryParse()/TryParseExact() methods should be used which return false instead of throwing exceptions.
• FormatException is more specific than Exception
• No need to check for Length == 8, because ParseExact()/TryParseExact() will do this
• "00:00:00" and "23:59:59" are not needed
• return true/false is you were able to parse, instead of throwing an exception (remember to check value returned from this method!)

Code:

private bool ValidateDatePeriod(string pdr, out DateTime startDate, 
						out DateTime endDate)
{
   string[] dates = pdr.Split('-');

   if (dates.Length != 2)
   {
	   return false;
   }

   // no need to check for Length == 8 because the following will do it anyway
   // no need for "00:00:00" or "23:59:59" either, I prefer AddDays(1)

   if(!DateTime.TryParseExact(dates[0], "yyyyMMdd", null, DateTimeStyles.None, out startDate))
	  return false;

   if(!DateTime.TryParseExact(dates[1], "yyyyMMdd", null, DateTimeStyles.None, out endDate))
	  return false;

   endDate = endDate.AddDays(1);
   return true;
}

I think we're doing it wrong. There is no such thing as the end of the day. AddTick(-1) only works under the convention that there are no time intervals smaller than a tick. Which is implementation dependent. Admittedly the question comes with a reference implementation, namely the .Net Framework DateTime class, but still we should take this as a clue that the function we really want is not EndOfDay() but StartOfNextDay()

public static DateTime StartOfNextDay(this DateTime date)
{
    return date.Date.AddDays(1);
}

For SQL Server (version 2008 R2 tested) this ranges works.

StarDate '2016-01-11 00:00:01.990' EndDate '2016-01-19 23:59:59.990'

Seems like ticks is greater that the last second of day and automatically round to next day. So i test and works, i made a dummy table with two dates for check what values is sql server catching and inserting in the stored procedure those parameters.

In Java 8, you can do it using LocalDate as follows:

    LocalDate localDateStart = LocalDate.now();
    Date startDate = Date.from(localDateStart.atStartOfDay(ZoneId.systemDefault()).toInstant());

    LocalDate localDateEnd = localDateStart.plusDays(1);
    Date endDate = Date.from(localDateEnd.atStartOfDay(ZoneId.systemDefault()).toInstant());