DateTimeParser with Timezone string conversion

DateTimeParser.cs

public static class DateTimeParser
    {
        private static readonly Dictionary<string, string> TimeZones = new Dictionary<string, string>()
        {
            {"CDT", "-0500"},
            {"CST", "-0600"},
            {"EDT", "-0400"},
            {"EST", "-0500"},
            {"PDT", "-0700"},
            {"PST", "-0800"},
            {"MST", "-0700"},
            {"MDT", "-0600"},
        };

        /// <summary>
        /// Parse a date string containing a timezone description (e.g. 01/22/2015 07:13 PM EST)
        /// Only common US timezones are supported.
        /// </summary>
        /// <param name="callDate"></param>
        /// <returns></returns>
        public static DateTimeOffset? ParseDateWithTimezone(string callDate)
        {
            DateTimeOffset result;
            callDate = TimeZones.Aggregate(callDate, (current, kvp) => current.Replace(kvp.Key, kvp.Value));
            if (DateTimeOffset.TryParse(callDate, out result))
                return result;
            return null;
        }
    }

TestDateTimeParser.cs

    [TestFixture]
    public class TestDateTimeParser
    {
        [Test]
        public void ParseValidDate()
        {
            var valid = "01/22/2015 07:13 PM EST";
            var result = DateTimeParser.ParseDateWithTimezone(valid);
            Assert.IsNotNull(result);

            Assert.AreEqual(2015, result.Value.UtcDateTime.Year);
            Assert.AreEqual(1, result.Value.UtcDateTime.Month);
            Assert.AreEqual(23, result.Value.UtcDateTime.Day);
            Assert.AreEqual(0, result.Value.UtcDateTime.Hour);
            Assert.AreEqual(13, result.Value.UtcDateTime.Minute);
            Assert.AreEqual(0, result.Value.UtcDateTime.Second);
        }

        [Test]
        public void ParseInvalidDateShouldReturnNull()
        {
            var invalid = "01/22/2015 07:13 PM CEDT";
            var result = DateTimeParser.ParseDateWithTimezone(invalid);
            Assert.IsNull(result);
        }
    }