diff --git a/da/preprocessing/era5.py b/da/preprocessing/era5.py
index 2d4a7aa1dde859174db6b4fd306dabf49e51311a..e5750f0263fde5f3d4c029756179e1861a381a45 100644
--- a/da/preprocessing/era5.py
+++ b/da/preprocessing/era5.py
@@ -132,7 +132,7 @@ def check_era_present(dacycle):
     NRUNYEARS_SPINUP = int(dacycle.dasystem['sib.spinup.years'])
 
     # SiB runs start 'NRUNYEARS_SPINUP'  years beforehand, on the first of the month
-    sib_starttime = dtm.datetime(starttime.year - NRUNYEARS_SPINUP, starttime.month, 1)
+    sib_starttime = dtm.datetime(starttime.year - NRUNYEARS_SPINUP, 1, 1)
     # SiB ends on the final day of the last month in the dasystem
     sib_endtime = dtm.datetime(endtime.year, endtime.month + 1, 1) - dtm.timedelta(days=1)