Question

I need to compare whether date is less than 3 months old.

I will get installation date:

DateTime installdate=DateTime.Parse("1/5/2012 8:12:14 PM");
if ((installdate<DateTime.Now.AddMonths(-3)))
{
  // do something
}

Is this the best way to compare the dates?

Thanks.

Answer 1

A few things to think about:

• “Is date x earlier than 3 months before today” isn’t the same as “today is more than 3 months later than date x”; you’ll need to make sure you have the exact semantics you want.
• Consider what you want to do with the time component – are you interested in dates or dates and times? (Would you expect the condition evaluation to change based on the current time of day?)
• Consider time zones: are you interested in “today in the system’s current time zone” or some fixed time zone?
• Depending on the source of the text data, you should possibly use DateTime.TryParse and you should possibly use DateTime.ParseExact or DateTime.TryParseExact, passing in the expected format (and culture)

Basically, there are various corner cases around date and time behaviour – you should explicitly think about all of these things (some of which are forced upon you if you use Noda Time instead of DateTime, btw 🙂

Regarding the first point, if the idea is that you get a trial period of three months from the installation date (or something similar), that suggests you should be adding three months to that instead.

I’d also change the variable name and get rid of the redundant parentheses, by the way:

DateTime installationDate = DateTime.Parse("1/5/2012 8:12:14 PM");
DateTime trialPeriodEnd = installationDate.AddMonths(3);
if (trialPeriodEnd > DateTime.Now)
{
   // do something
}

Assuming you’re storing the installation date yourself somewhere, I would try to store it in some form which is less ambiguous – possibly even storing just a “ticks” value instead of a string. But assuming you are storing it yourself, you shouldn’t need to use TryParse – it makes sense to go “bang” if you can’t parse the value. I’d use ParseExact, probably with a standard format specifier of “o” (round trip).