diff --git a/dcaf/shared/time.py b/dcaf/shared/time.py
index 4c1da22..c658ea6 100644
--- a/dcaf/shared/time.py
+++ b/dcaf/shared/time.py
@@ -24,7 +24,15 @@
 from dcaf.shared.validation import validate_non_negative
 
 
-_FLOAT_TOLERANCE = 1e-12
+# A floating point absolute error tolerance of 1e-8 is chosen because this is on the order
+# of <1 second over a year
+#       1e-8 * 365 days = 0.315 sec
+# and <1 ms over a day
+#       1e-8 * 1 day = 0.864 ms
+# While floating point arithmetic has several orders of magnitude more precision than 1e-8,
+# this is already a very strict tolerance for the domain setting; DCAF uses the day as the
+# base unit of time.
+_FLOAT_TOLERANCE = 1e-8
 
 
 class PeriodTruncationWarning(UserWarning):
diff --git a/tests/unit/test_time_helpers.py b/tests/unit/test_time_helpers.py
index a292d2c..76b8560 100644
--- a/tests/unit/test_time_helpers.py
+++ b/tests/unit/test_time_helpers.py
@@ -57,6 +57,8 @@ def test_is_month_end_when_next_day_is_start_of_next_month(day):
 
 
 # === _whole_days_with_truncation ===
+# NOTE: The function implementation uses a floating point tolerance of 1e-8, corresponding to
+#       <1 second per year and <1 ms per day.
 
 
 @pytest.mark.parametrize(
@@ -66,8 +68,10 @@ def test_is_month_end_when_next_day_is_start_of_next_month(day):
         (15.5, (15, True)),
         (0.5, (0, True)),
         (0.0, (0, False)),
-        (14.0 + 1e-13, (14, False)),  # within tolerance of a whole day
-        (14.0 - 1e-13, (14, False)),
+        (14.0 + 1e-12, (14, False)),  # within tolerance of a whole day
+        (14.0 - 1e-12, (14, False)),
+        (14.0 - 5e-9, (14, False)),  # just inside the 1e-8 tolerance
+        (14.0 - 2e-8, (13, True)),  # just outside the 1e-8 tolerance
         (14.0 - 1e-6, (13, True)),  # outside tolerance, so the partial day is dropped
         (14.0 + 1e-6, (14, True)),  # outside tolerance, so the partial day is dropped
     ],