diff --git a/ecgtools/parsers/cmip.py b/ecgtools/parsers/cmip.py
index 1e3d820..18d432a 100644
--- a/ecgtools/parsers/cmip.py
+++ b/ecgtools/parsers/cmip.py
@@ -74,6 +74,10 @@ def parse_cmip6(file):
             attributes['init_year'] = init_year
             attributes['start_time'] = start_time
             attributes['end_time'] = end_time
+            if not (start_time and end_time):
+                attributes['time_range'] = None
+            else:
+                attributes['time_range'] = f'{start_time}-{end_time}'
             attributes['path'] = file
         return attributes