Parse datetime in multiple formats

One way to deal with this problem would be setting up a factory method that "understands" different formats, and parses them accordingly.

You can create a chain of if-then-elses to deal with this problem, but you can also make a "table-driven" implementation: what you need is an array of delegates that take a string, and tell you two things:

• Whether or not this delegate can parse the incoming string, and
• If yes, what is the result of that parse, expressed as DateTime

Here is a sample implementation:

private static readonly DateParsers = new Func<string,Tuple<DateTime,bool>>[] {
    (s) => {
        long res;
        if (long.TryParse(s, out res)) {
            // The format was correct - make a DateTime,
            // and return true to indicate a successful parse
            return Tuple.Create(new DateTime(res), true);
        } else {
            // It does not matter what you put in the Item1
            // when Item2 of the tuple is set to false
            return Tuple.Create(DateTime.MinValue, false);
        }
    }
    ...
    // Add similar delegates for other formats here
};

Now your factory method could be implemented as follows:

private static bool TryParseMultiformat(string s, out DateTime res) {
    // Check all parsers in turn, looking for one returning success
    foreach (var p in DateParsers) {
        var tmp = p(s);
        if (tmp.Item2) {
            res = tmp.Item1;
            return true;
        }
    }
    res = DateTime.MinValue;
    return false;
}

You are looking for the DateTime.ParseExact (MSDN Article)

Which you would use in a situation like this:

string[] formats= { "MM/dd/yyyy hh:mm:ss tt", "yyyy-MM-dd hh:mm:ss" }
var dateTime = DateTime.ParseExact("07/25/2013 6:37:31 PM", formats, new CultureInfo("en-GB"), DateTimeStyles.None);

This allows you to add as many DateTime formats to the array as you need and the method will do the conversion without the if...else statements.

If your integer is in seconds since Unix Epoch you add the number of seconds to the DateTime of the Epoch (01/01/1970) (.Net doesn't have an out of the box method for this, but the logic is seconds since 'Epoch'):

new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddSeconds(seconds);

From this question.

You should consider requiring a timezone. 1 doesn't need it, but #2 and #3 do.

public DateTime ParseRequestDate()
{
    // https://stackoverflow.com/questions/2883576/how-do-you-convert-epoch-time-in-c

    CultureInfo enUS = new CultureInfo("en-US");

    var dt = "1374755180";
    //var dt = "7/25/2013 6:37:31 PM";
    //var dt = "2013-07-25 14:26:00";

    DateTime dateValue;
    long dtLong;

    // Scenario #1
    if (long.TryParse(dt, out dtLong))
        return dtLong.FromUnixTime();

    // Scenario #2
    if (DateTime.TryParseExact(dt, "MM/dd/yyyy hh:mm:ss tt", enUS, DateTimeStyles.None, out dateValue))
        return dateValue;

    // Scenario #3
    if (DateTime.TryParseExact(dt, "yyyy-MM-dd hh:mm:ss", enUS, DateTimeStyles.None, out dateValue))
        return dateValue;

    throw new SomeException("Don't know how to parse...");
}

EDIT As Matt Johnson points out, DateTime.TryParseExact accepts an array of format strings. 2 & 3 could be condensed.

public DateTime ParseRequestDate()
{
    // https://stackoverflow.com/questions/2883576/how-do-you-convert-epoch-time-in-c

    CultureInfo enUS = new CultureInfo("en-US");

    var dt = "1374755180";
    //var dt = "7/25/2013 6:37:31 PM";
    //var dt = "2013-07-25 14:26:00";

    DateTime dateValue;
    long dtLong;

    // Scenario #1
    if (long.TryParse(dt, out dtLong))
        return dtLong.FromUnixTime();

    // Scenario #2 & #3
    var formatStrings = new string[] { "MM/dd/yyyy hh:mm:ss tt", "yyyy-MM-dd hh:mm:ss" };
    if (DateTime.TryParseExact(dt, formatStrings, enUS, DateTimeStyles.None, out dateValue))
        return dateValue;

    throw new SomeException("Don't know how to parse...");
}

The epoch conversion I borrowed from another question. (An extension method)

public static class MyExtensions
{
    public static DateTime FromUnixTime(this long unixTime)
    {
        var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
        return epoch.AddSeconds(unixTime);
    }
}