What is the best way to store DateTime into cookies?

Question

I am working on an ASP .NET project, and currently I am using this code to store and retrieve DateTime on cookies:

HttpCookie cookie = new HttpCookie("myCookie");
if (Request.Cookies["myCookie"] != null)
{
    cookie = Request.Cookies["myCookie"];
}
else
{
    cookie["From"] = DateTime.Now.AddDays(-7).ToShortDateString();
    cookie["To"] = DateTime.Now.ToShortDateString();
    Response.Cookies.Add(cookie);
}
model.FromDate = DateTime.Parse(cookie["From"]);
model.ToDate = DateTime.Parse(cookie["To"]);

And in my View I am using Razor to recovery the model values like this:

<input type="text" id="from" value="@Model.FromDate.ToShortDateString()" readonly="readonly" />
<input type="text" id="to" value="@Model.ToDate.ToShortDateString()" readonly="readonly" />

It is working fine when I run it locally, but when I uploaded to production, when recovering the DateTime from cookie, the date is changed in one day. For example, I selected a date range from 3/25/2016 to 4/1/2016, then I have gone to another page, and when I came back to this page, the page showed a date range from 3/24/2016 to 3/31/2016, decreased in one day.

Do you know what I am doing wrong here, why this just happens in production (I suppose is something related to server date) and, finally, what is the best way to store and retrieve a DateTime on cookie?

Answer 1

You can store the date in Ticks

cookie["From"] = DateTime.Now.AddDays(-7).Ticks.ToString();
cookie["To"] = DateTime.Now.Ticks.ToString();

and recover like this:

model.FromDate = new DateTime(Convert.ToInt64(cookie["From"]));
model.ToDate = new DateTime(Convert.ToInt64(cookie["To"]));

Hope this helps you

Answer 2

When storing date / time as string you should always consider timezones. I recommend you to use DateTimeOffset instead of DateTime :

var now = DateTimeOffset.Now;
var asString = now.ToString(CultureInfo.InvariantCulture);
var asDatetimeOffset = DateTimeOffset.Parse(asString, CultureInfo.InvariantCulture);

The string looks like this: 04/01/2016 22:01:09 +02:00

(You need to know the client's timezone to correctly calculate its 'local' time. In your example, the server uses it's own time.)

The result of DateTime.Parse(cookie["From"]); is set to DateTimeKind.Unspecified . Any further operation (like AddDays) depends the system's timezone.

I think you should specify the culture and tell the parser which DateTimeKind to expect:

model.FromDate = DateTime.Parse(cookie["From"], CultureInfo.CurrentCulture, DateTimeStyles.AssumeLocal);
model.ToDate = DateTime.Parse(cookie["To"], CultureInfo.CurrentCulture, DateTimeStyles.AssumeLocal);

Note : DateTime.ToShortDateString isn't a good choice, since it is defined by the current culture's DateTimeFormatInfo object.