How important is it to use a variable for DateTime.Today when concerned about performance?

Question

I just saw this upvoted comment

IIRC DateTime.Today is a quite expensive call, so you better store the value in a variable first.

It was in response to a post that contained the code:

var first = 
    new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddMonths(-1);
var last = 
    new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddDays(-1);

If I am looking to improve performance, how important is it to store DateTime.Today in a variable instead of calling it multiple times? And roughly how many uses of DateTime.Today would justify creating a variable for it?

Edit: I realize I should test my program to see if there are performance problems first before worrying about something as trivial as this. For the sake of this question, assume that I have already done this and determined that additional optimization is needed.

Answer 1

how important is it to store DateTime.Today in a variable

The best answer to that is to benchmark on the hardware you expect your code to be running on. Unless you are calling it in an extremely tight loop, I doubt it will be a problem.

A better reason to store it in a variable is that you just might roll over from one day to the next between the two calls.

UPDATE

To provide an order of magnitude, @RichardBrown shared in his answer a link indicating that the cost of DateTime.Today is was tested to be on the order of a few hundred nanoseconds (on the particular hardware used for that test).

Answer 2

Benchmark (on my machine, using the Stopwatch class):

10,000 DateTime.Today calls and assignment to local variable: 0.0125781 seconds.

10,000 Assignment only operations: 0.0001062 seconds.

Code:

var s = new Stopwatch();
DateTime date = DateTime.Today;
DateTime date2 = DateTime.Today;
s.Start();
for (int i=0; i<10000; i++)
    date = DateTime.Today;
s.Stop();
Debug.Print(s.Elapsed.ToString());

s.Reset();
s.Start();
for (int i=0; i<10000; i++)
    date2 = date;
s.Stop();
Debug.Print(s.Elapsed.ToString());

Answer 3

I reject the premise that DateTime.Today is an expensive call. You should store it in a variable if it's important that it not change over time. If this code runs right around midnight of the end of a month you could have...problems. From a performance perspective I highly doubt this would be an issue.

In any case, it would be a micro-optimization. As with any performance issues if your working program is too slow you should profile it and look for sections taking quite a lot of time and focus on optimizing those, looking for code spending more time running than you would expect. If it happens that this one line of code is consuming a lot of time then consider changing it. Until then, only refactor it for correctness or readability, not performance.

Answer 4

For hard numbers on the performance of DateTime read this blog post. As previous answers state it's important to look at your particular configuration and needs when determining performance.

Answer 5

I would guess that it is a problem if you are calling the code in a loop. But the best way to find out is to time it and see for yourself how long it takes.

var sw = new Stopwatch();
sw.Start();
today = DateTime.Today();
sw.Stop();
var ts = stopWatch.Elapsed;

MSDN Stopwatch reference

Answer 6

The real reason to extract DateTime.Today as variable is to prevent possible bugs which will appear because of Murphy's law, but would never be isolated and fixed..

Idea is that between first and second usage of Today actual date may be changed, so you can get, for example, previous year and new month here:

var first = new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddMonths(-1);

which may result, for example, in (January 2013) - 1 month instead of (December 2013) - 1 month.

More probable and more evil issue will be if you get date change between first and last assignment:

var first = new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddMonths(-1);
var last = new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1).AddDays(-1);

which will result a year-more period.. Depending on your logic, it may lead to really expensive loss.

Answer 7

More important than the question of performance is imo the readability and maintainability of code.

unless you don't have performance issues I would follow the rule "pay attention to performance improvements"

Perhaps if you have a loop than a variable could be named appropriate with "today" but again as long as there arent performance issued i would focus on the other concerns