diff --git a/noaweather/metar.py b/noaweather/metar.py
index a871e56..739ae79 100644
--- a/noaweather/metar.py
+++ b/noaweather/metar.py
@@ -129,7 +129,7 @@ def updateMetar(self, db, path):
         INSBUF = cursor.arraysize
 
         today_prefix = datetime.utcnow().strftime('%Y%m')
-        yesterday_prefix = (datetime.utcnow() - timedelta(days=-1)).strftime('%Y%m')
+        yesterday_prefix = (datetime.utcnow() + timedelta(days=-1)).strftime('%Y%m')
 
         today = datetime.utcnow().strftime('%d')