Calculating past datetime in C#

I am working on an algorithm in C# to calculate a past DateTime based on an input string with the following characteristics:

The string contains an integer followed by either 'D', 'M' or 'Y', such as "1D" or "90M".

The output will be DateTime.Now minus the corresponding number of days, months or years.

The issue I am having is that if, for instance, I switch the input string on a Regex (D, M or Y) and subtract the corresponding TimeSpan from DateTime.Now, the new TimeSpan() constructor does not accept months or years, only days.

if (new Regex(@"[0-9]+D").IsMatch(value))
{
    newDate = DateTime.Now - TimeSpan(Int32.Parse(value.Replace("D", "")), 0, 0);
}

This logic is fine if the input string is in days, but the constructor for Tim开发者_如何学编程eSpan does not accept months or years, and it would be incredibly inaccurate if I assumed each month had 30 days, or each year had 365 days.

Does anyone have thoughts on how to implement this algorithm?

Thanks!

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DateTime has AddMonths, AddDays and AddYears methods. Use them with minus to substract

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Could you not rather try using the AddDays/AddMonths/AddYears but with negative numbers?

From DateTime.AddDays Method

The value parameter can be negative or positive.

And then maybe just implement a switch stament to apply the appropriate Add method.

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To subtract months, I create a new DateTime and just evaluate month/year. So 1/2010 - 6 months would be 6/2010... once you have the month/year established, you can look at the original datetime day component, and ensure it fits within the month.

That's what I did. Year was evaluated the same way. Subtracting days is easy; use the TimeSpan component to do it.

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Remember that you can add negative amounts as well and check out this method and this one.

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http://msdn.microsoft.com/en-us/library/3z48198e.aspx TimeSpan.TryParse accepts very close to your string as long as you can fits its formatting OR convert from yours to its.