GetTimePeriod.cs

public static TimeSpan GetTimePeriod(string str)
{
	if (str == null)
		throw new ArgumentNullException("str");

	// Plain old regex: ([\+-]?)(\d+)([A-Za-z]+)?
	Regex re = new Regex("(?<Operator>[+\\-]?)(?<Value>\\d+)(?<Unit>[A-Za-z]+)?", RegexOptions.IgnoreCase);
	Match m = re.Match(str);

	if (m.Success)
	{
		var op = m.Groups["Operator"].Value;
		var unit = m.Groups["Unit"].Value.ToLower();
		var value = int.Parse(m.Groups["Value"].Value);
		var multiplier = op == "-" ? -1 : 1;

		switch (unit)
		{
			case "d":
			case "day":
			case "days":
				return new TimeSpan(value * multiplier, 0, 0, 0);
			case "h":
			case "hour":
			case "hours":
				return new TimeSpan(0, value * multiplier, 0, 0);
			case "m":
			case "minute":
			case "minutes":
				return new TimeSpan(0, 0, value * multiplier, 0);
			default:
				// assume hours
				return new TimeSpan(0, value * multiplier, 0, 0);
		}
	}
	return TimeSpan.Zero;
}