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diff --git a/date-fns/src/_lib/startOfUTCISOWeek/test.js b/date-fns/src/_lib/startOfUTCISOWeek/test.js
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+++ b/date-fns/src/_lib/startOfUTCISOWeek/test.js
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+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import startOfUTCISOWeek from '.'
+
+describe('startOfUTCISOWeek', function() {
+  it('returns the date with the time set to 00:00:00 and the date set to the first day of an ISO week', function() {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    var result = startOfUTCISOWeek(date)
+    assert.deepEqual(result, new Date(Date.UTC(2014, 8 /* Sep */, 1)))
+  })
+
+  it('accepts a timestamp', function() {
+    var date = new Date(Date.UTC(2014, 1 /* Feb */, 11, 11, 55, 0)).getTime()
+    var result = startOfUTCISOWeek(date)
+    assert.deepEqual(result, new Date(Date.UTC(2014, 1 /* Feb */, 10)))
+  })
+
+  it('does not mutate the original date', function() {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    startOfUTCISOWeek(date)
+    assert.deepEqual(date, new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0)))
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    var result = startOfUTCISOWeek(new Date(NaN))
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function() {
+    assert.throws(startOfUTCISOWeek.bind(null), TypeError)
+  })
+})