importexp: fix missing variable reference in INITIAL/FINAL_DATE initialization

importexp

@@ -133,8 +133,8 @@ ${couplingTimeStep:+COUPLER_TIME_STEP = $couplingTimeStep}
 ###/
 EOF
 
-    INITIAL_DATE=${initial_date:-start_date}
-    FINAL_DATE=${final_date:-end_date}
+    INITIAL_DATE=${initial_date:-$start_date}
+    FINAL_DATE=${final_date:-$end_date}
 
     $BINDIR/namelist2config -i "$EXPNAME" -a "$atmos_gridID" -A "$atmos_refinement" -o "$ocean_gridID" -O "$ocean_refinement" -I "$INITIAL_DATE" -F "$FINAL_DATE" $LOG_FILE
 ###     i => 'EXP_ID',