diff --git a/gnssanalysis/filenames.py b/gnssanalysis/filenames.py
index 8c7cf3f..01b5329 100644
--- a/gnssanalysis/filenames.py
+++ b/gnssanalysis/filenames.py
@@ -370,12 +370,20 @@ def generate_IGS_nominal_span(start_epoch: datetime.datetime, end_epoch: datetim
     return nominal_span_string(span)
 
 
-def nominal_span_string(span_seconds: float) -> str:
+def nominal_span_string(span_seconds: float, igs_week_as_days: bool = True) -> str:
     """Generate the 3 character LEN or SMP string for IGS filenames based on total span seconds
 
     :param float span_seconds: Number of seconds in span of interest
+    :param bool igs_week_as_days: Use IGS common convention of expressing 1 week as 07D rather than 01W (in contrast
+        to IGS long filename spec which says to use the largest unit, i.e. 01W). Default: True (07D)
     :return str: 3 character span string as per IGS standard
     """
+
+    if span_seconds is None or span_seconds == 0:
+        return "00U"
+
+    if span_seconds < 0:
+        raise ValueError(f"Span seconds can't be negative: {span_seconds}")
     # A year is ambiguous, for our purposes we want 365 (not 365.25 or 366) days, as we
     # use it as a lower bound
     sec_in_year = 365 * gn_const.SEC_IN_DAY
@@ -397,7 +405,13 @@ def nominal_span_string(span_seconds: float) -> str:
         num_weeks = int(span_seconds // gn_const.SEC_IN_WEEK)
         # IGS uses 07D to represent a week
         # TODO: Handle JPL - uses 01W for a week
-        if (span_seconds % gn_const.SEC_IN_WEEK) < gn_const.SEC_IN_DAY and num_weeks > 1:
+        # We stick to outputting in weeks if either:
+        #  - there are >=14 days (>1 week with int division) or
+        #  - there are >=7 days (>0 weeks with int division), but ONLY if we are NOT applying IGS (inconsistent)
+        #    convention of outputting 1 week as 07D
+        if (span_seconds % gn_const.SEC_IN_WEEK) < gn_const.SEC_IN_DAY and (
+            num_weeks > 1 or (num_weeks > 0 and igs_week_as_days == False)
+        ):
             unit = "W"
             span_unit_counts = num_weeks
         else:
@@ -440,7 +454,7 @@ def convert_nominal_span(
 ) -> Union[datetime.timedelta, None]:
     """Effectively invert :func: `filenames.generate_nominal_span`, turn a span string into a timedelta
 
-    :param str nominal_span: Three-character span string in IGS format (e.g. 01D, 15M, 01L ?)
+    :param str nominal_span: Three-character span string in IGS format (e.g. 01D, 15M, 01L)
     :param Literal["none", "timedelta"] non_timed_span_output: when a non-timed span e.g. '00U' is encountered,
         return a zero-length timedelta (default), or return None.
         instead of raising / warning / returning zero-length timedelta.
diff --git a/tests/test_filenames.py b/tests/test_filenames.py
index b7c40b9..eeace0e 100644
--- a/tests/test_filenames.py
+++ b/tests/test_filenames.py
@@ -266,3 +266,44 @@ def test_convert_nominal_span(self):
 
         with self.assertRaises(ValueError):
             filenames.convert_nominal_span("005M")
+
+    def test_nominal_span_string(self):
+
+        # Standard conversions
+
+        # 15 sec
+        self.assertEqual(filenames.nominal_span_string(15), "15S")
+
+        # 5 mins
+        self.assertEqual(filenames.nominal_span_string(300), "05M")
+        self.assertEqual(filenames.nominal_span_string(300.0), "05M")
+
+        # 6 hours
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 6), "06H")
+
+        # One day
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 24), "01D")
+
+        # 36 hours must be expressed as hours not days, to preserve precision
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 36), "36H")
+
+        # 2 days
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 24 * 2), "02D")
+
+        # 1 week (we follow IGS' lead in expressing this as 07D, despite the IGS long filename spec saying it
+        # should be 01W)
+        # By default, IGS style weeks: 07D
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 24 * 7), "07D")
+
+        # Optionally, format compliant weeks: 01W
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 24 * 7, igs_week_as_days=False), "01W")
+
+        # 1 lunar cycle (28 days) # TODO note: currently unsupported. Will come out as '04W'
+        self.assertEqual(filenames.nominal_span_string(60 * 60 * 24 * 28), "04W")
+
+        # Ensure smaller unit is used to preserve presision when needed.
+        self.assertEqual(filenames.nominal_span_string(65), "65S")
+
+        # Up till the point it would be too big, then we 'up-shift' unit, and round up to the nearest value
+        # TODO this isn't currently supported
+        # self.assertEqual(filenames.nominal_span_string(110), "02M")