diff --git a/nextstrain_profiles/nextstrain-gisaid/builds.yaml b/nextstrain_profiles/nextstrain-gisaid/builds.yaml
index 170963e80..ae9c22f65 100644
--- a/nextstrain_profiles/nextstrain-gisaid/builds.yaml
+++ b/nextstrain_profiles/nextstrain-gisaid/builds.yaml
@@ -800,47 +800,47 @@ colors:
   default:
     clade_recency: "all"
   global_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   global_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   global_6m:
-    clade_recency: 6
+    clade_recency: "6M"
   africa_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   africa_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   africa_6m:
-    clade_recency: 6
+    clade_recency: "6M"
   asia_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   asia_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   asia_6m:
-    clade_recency: 6
+    clade_recency: "6M"
   europe_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   europe_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   europe_6m:
-    clade_recency: 6
+    clade_recency: "6M"
   north-america_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   north-america_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   north-america_6m:
-    clade_recency: 6
+    clade_recency: "6M"
   oceania_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   oceania_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   oceania_6m:
-    clade_recency: 6
+    clade_recency: "6M"
   south-america_1m:
-    clade_recency: 1
+    clade_recency: "1M"
   south-america_2m:
-    clade_recency: 2
+    clade_recency: "2M"
   south-america_6m:
-    clade_recency: 6
+    clade_recency: "6M"
 
 # if different traits should be reconstructed for some builds, specify here
 # otherwise the default trait config in defaults/parameters.yaml will used
diff --git a/scripts/assign-colors.py b/scripts/assign-colors.py
index b0624340a..679a77d3f 100644
--- a/scripts/assign-colors.py
+++ b/scripts/assign-colors.py
@@ -1,6 +1,7 @@
 import argparse
+import datetime
+import isodate
 import pandas as pd
-from datetime import datetime, timedelta
 
 # Forced colours MUST NOT appear in the ordering TSV
 forced_colors = {
@@ -10,11 +11,27 @@ def date_within_last_n_months(date_str, cutoff_date):
     if 'XX' in date_str:
         return False  # Ignore uncertain dates
     try:
-        date = datetime.strptime(date_str, "%Y-%m-%d")
+        date = datetime.datetime.strptime(date_str, "%Y-%m-%d")
         return date >= cutoff_date
     except ValueError:
         return False
 
+
+def relative_date(duration: str):
+    """
+    Convert an ISO 8601 duration to an absolute date by subtracting it from the
+    current date.
+
+    `duration` should be a backwards-looking relative date in ISO 8601 duration
+    format with optional P prefix (e.g. '1W', 'P1W').
+    """
+    if duration.startswith('P'):
+        duration = duration
+    else:
+        duration = 'P' + duration
+    return datetime.date.today() - isodate.parse_duration(duration)
+
+
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(
         description="Assign colors based on ordering",
@@ -25,7 +42,10 @@ def date_within_last_n_months(date_str, cutoff_date):
     parser.add_argument('--color-schemes', type=str, required=True, help="input color schemes file")
     parser.add_argument('--metadata', type=str, help="if provided, restrict colors to only those found in metadata")
     parser.add_argument('--clade-node-data', type=str, help="if provided, restrict to only those clades found in tree")
-    parser.add_argument('--clade-recency', type=int, help="if provided, restrict to clades found in tree within X months of present")
+    parser.add_argument('--clade-recency', type=relative_date, metavar='DURATION',
+        help="""if provided, restrict to clades found in tree within this time
+             frame. Format: ISO 8601 duration with optional P prefix (e.g. '1W',
+             'P1W')""")
     parser.add_argument('--output', type=str, required=True, help="output colors tsv")
     args = parser.parse_args()
 
@@ -62,16 +82,13 @@ def date_within_last_n_months(date_str, cutoff_date):
             clades = json.load(fh)['nodes']
 
         if args.clade_recency is not None and args.metadata:
-            # Calculate the cutoff date based on clade_recency (number of months ago from today)
-            cutoff_date = datetime.today() - timedelta(days=args.clade_recency * 30)  # approximate months as 30 days
-
             # Generate a set of present values within the specified recency
             subset_present = set()
             metadata = pd.read_csv(args.metadata, delimiter='\t')
             for strain, info in clades.items():
                 if strain in metadata['strain'].values:
                     date_str = metadata.loc[metadata['strain'] == strain, 'date'].values[0]
-                    if date_within_last_n_months(date_str, cutoff_date):
+                    if date_within_last_n_months(date_str, args.clade_recency):
                         subset_present.add(info["clade_membership"])
 
             # Restrict to only those present while maintaining order