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diff --git a/date-fns/src/_lib/getTimezoneOffsetInMilliseconds/test.js b/date-fns/src/_lib/getTimezoneOffsetInMilliseconds/test.js
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+++ b/date-fns/src/_lib/getTimezoneOffsetInMilliseconds/test.js
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+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import getTimezoneOffsetInMilliseconds from '.'
+
+describe('getTimezoneOffsetInMilliseconds', function () {
+  it('works for a modern date', function () {
+    var date = new Date(2018, 0 /* Jan */, 1, 12, 34, 56, 789)
+    var result = date.getTime() - getTimezoneOffsetInMilliseconds(date)
+    var expectedResult = Date.UTC(2018, 0 /* Jan */, 1, 12, 34, 56, 789)
+    assert(result === expectedResult)
+  })
+
+  it('works for a date before standardized timezones', function () {
+    var date = new Date(1800, 0 /* Jan */, 1, 12, 34, 56, 789)
+    var result = date.getTime() - getTimezoneOffsetInMilliseconds(date)
+    var expectedResult = Date.UTC(1800, 0 /* Jan */, 1, 12, 34, 56, 789)
+    assert(result === expectedResult)
+  })
+
+  it('works for a date BC', function () {
+    var date = new Date(-500, 0 /* Jan */, 1, 12, 34, 56, 789)
+    var result = date.getTime() - getTimezoneOffsetInMilliseconds(date)
+    var expectedResult = Date.UTC(-500, 0 /* Jan */, 1, 12, 34, 56, 789)
+    assert(result === expectedResult)
+  })
+})