diff --git a/docs/data-model.rst b/docs/data-model.rst
index 72a5129534..c445ae6f5e 100644
--- a/docs/data-model.rst
+++ b/docs/data-model.rst
@@ -634,12 +634,12 @@ requirements for a valid set of mutations are:
 
 - ``site`` must refer to a valid site ID;
 - ``node`` must refer to a valid node ID;
-- ``time`` must either be UNKNOWN_TIME (a NAN value which indicates
+- ``time`` must either be ``UNKNOWN_TIME`` (a NAN value which indicates
   the time is unknown) or be a finite value which is greater or equal to the
   mutation ``node``'s ``time``, less than the ``node`` above the mutation's
   ``time`` and equal to or less than the ``time`` of the ``parent`` mutation
-  if this mutation has one. If one mutation on a site has UNKNOWN_TIME then all mutations
-  at that site must, a mixture of known and unknown is not valid.
+  if this mutation has one. If one mutation on a site has ``UNKNOWN_TIME`` then
+  all mutations at that site must, as a mixture of known and unknown is not valid.
 - ``parent`` must either be the null ID (-1) or a valid mutation ID within the
   current table
 
@@ -668,6 +668,9 @@ mutation does not result in any change of state. This error is
 raised at run-time when we reconstruct sample genotypes, for example
 in the :meth:`TreeSequence.variants` iterator.
 
+.. note:: As ``tskit.UNKNOWN_TIME`` is implemented as a ``NaN`` value, tests for
+    equality will always fail. Use ``tskit.is_unknown_time`` to detect unknown
+    values.
 
 .. _sec_migration_requirements:
 
diff --git a/python/CHANGELOG.rst b/python/CHANGELOG.rst
index 644be33d84..99981b7b02 100644
--- a/python/CHANGELOG.rst
+++ b/python/CHANGELOG.rst
@@ -28,6 +28,8 @@
 
 - New ``tree.is_isolated(u)`` method (:user:`hyanwong`, :pr:`443`).
 
+- ``tskit.is_unknown_time`` can now check arrays. (:user:`benjeffery`, :pr:`857`).
+
 --------------------
 [0.3.1] - 2020-09-04
 --------------------
diff --git a/python/tests/test_util.py b/python/tests/test_util.py
index 657ed293cc..151365d145 100644
--- a/python/tests/test_util.py
+++ b/python/tests/test_util.py
@@ -29,6 +29,7 @@
 import unittest
 
 import numpy as np
+from numpy.testing import assert_array_equal
 
 import tests.tsutil as tsutil
 import tskit.util as util
@@ -64,7 +65,15 @@ def test_canonical_json(self):
 
 
 class TestUnknownTime(unittest.TestCase):
-    def test_unknown_time(self):
+    def test_unknown_time_bad_types(self):
+        with self.assertRaises(ValueError):
+            util.is_unknown_time("bad")
+        with self.assertRaises(ValueError):
+            util.is_unknown_time(np.array(["bad"]))
+        with self.assertRaises(ValueError):
+            util.is_unknown_time(["bad"])
+
+    def test_unknown_time_scalar(self):
         self.assertTrue(math.isnan(UNKNOWN_TIME))
         self.assertTrue(util.is_unknown_time(UNKNOWN_TIME))
         self.assertFalse(util.is_unknown_time(math.nan))
@@ -72,6 +81,28 @@ def test_unknown_time(self):
         self.assertFalse(util.is_unknown_time(0))
         self.assertFalse(util.is_unknown_time(math.inf))
         self.assertFalse(util.is_unknown_time(1))
+        self.assertFalse(util.is_unknown_time(None))
+        self.assertFalse(util.is_unknown_time([None]))
+
+    def test_unknown_time_array(self):
+        test_arrays = (
+            [],
+            [True],
+            [False],
+            [True, False] * 5,
+            [[True], [False]],
+            [[[True, False], [True, False]], [[False, True], [True, False]]],
+        )
+        for spec in test_arrays:
+            spec = np.asarray(spec, dtype=bool)
+            array = np.zeros(shape=spec.shape)
+            array[spec] = UNKNOWN_TIME
+            assert_array_equal(spec, util.is_unknown_time(array))
+
+        weird_array = [0, UNKNOWN_TIME, np.nan, 1, math.inf]
+        assert_array_equal(
+            [False, True, False, False, False], util.is_unknown_time(weird_array)
+        )
 
 
 class TestNumpyArrayCasting(unittest.TestCase):
diff --git a/python/tskit/util.py b/python/tskit/util.py
index 1b145d4b72..3b0e5cbcb3 100644
--- a/python/tskit/util.py
+++ b/python/tskit/util.py
@@ -23,7 +23,6 @@
 Module responsible for various utility functions used in other modules.
 """
 import json
-import struct
 
 import numpy as np
 
@@ -45,9 +44,16 @@ def canonical_json(obj):
 def is_unknown_time(time):
     """
     As the default unknown mutation time is NAN equality always fails. This
-    method compares the bitfield.
+    method compares the bitfield such that unknown times can be detected.
+    Either single floats can be passed or lists/arrays.
+
+    :param float or array-like time: Value or array to check.
+    :return: A single boolean or array of booleans the same shape as ``time``.
+    :rtype: bool or np.array(dtype=bool)
     """
-    return struct.pack(">d", UNKNOWN_TIME) == struct.pack(">d", time)
+    return np.asarray(time, dtype=np.float64).view(np.uint64) == np.float64(
+        UNKNOWN_TIME
+    ).view(np.uint64)
 
 
 def safe_np_int_cast(int_array, dtype, copy=False):