Back in 1980, Daylight saving time (DST) started on April 27th. But calling the IsDaylightSavingTime method in System.TimeZoneInfo class for April 15, 1980 returns true. The following test fails:

  1. [TestMethod]
  2. public void DST_Started_On_April_27_1980()
  3. {
  4.     var ts = new DateTime(1980, 4, 15, 12, 0, 0);
  5.     var isDst = Utils.EasternTimeZone.IsDaylightSavingTime(ts);
  6.     Assert.IsFalse(isDst);
  7. }

Let’s do some sleuthing and get to the bottom of this.

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