diff --git a/libpurple_to_markdown/__main__.py b/libpurple_to_markdown/__main__.py
index 79079b7..a060e5d 100644
--- a/libpurple_to_markdown/__main__.py
+++ b/libpurple_to_markdown/__main__.py
@@ -8,12 +8,13 @@ from . import (format_messages, merge_adjacent_messages,
 
 logger = logging.getLogger(__name__)
 
-def group_messages_by_year(messages: list[Message]) -> dict[int, list[Message]]:
-    by_year = {}
+def group_messages_by_period(messages: list[Message]) -> dict[str, list[Message]]:
+    by_period: dict[str,list[Message]] = {}
     for msg in messages:
-        by_year.setdefault(msg.sent_at.year, []).append(msg)
+        period_key = f'{msg.sent_at.year}-{msg.sent_at.month:02}'
+        by_period.setdefault(period_key, []).append(msg)
         del msg
-    return by_year
+    return by_period
 
 def parse_args():
     parser = argparse.ArgumentParser()
@@ -34,15 +35,15 @@ def main():
     all_messages = filter_useless_messages(all_messages)
     all_messages = merge_adjacent_messages(all_messages)
 
-    messages_by_year = group_messages_by_year(all_messages)
+    messages_by_period = group_messages_by_period(all_messages)
 
-    for year, messages in messages_by_year.items():
-        output_file = args.output / f'{server} - {receipient} - {year}.md'
-        logger.info("Writing %d messages to %s", len(messages), output_file)
+    for period_key, messages in messages_by_period.items():
+        output_file = args.output / f'{server} - {receipient} - {period_key}.md'
+        logger.info("Writing % 5d messages to %s", len(messages), output_file)
         with open(output_file, 'w') as f:
-            f.write(format_messages(messages, title = f'{server} - {receipient} - {year}'))
+            f.write(format_messages(messages, title = f'{server} - {receipient} - {period_key}'))
 
-        del year, messages, output_file
+        del period_key, messages, output_file
 
 
 if __name__ == '__main__':