diff --git a/src/write_xlsx.c b/src/write_xlsx.c
index fd590cf..1c1ef92 100644
--- a/src/write_xlsx.c
+++ b/src/write_xlsx.c
@@ -180,9 +180,9 @@ SEXP C_write_data_frame_list(SEXP df_list,
       if(!Rf_isMatrix(COL) && !Rf_inherits(COL, "data.frame"))
         rows = max(rows, Rf_length(COL));
       if(coltypes[i] == COL_DATE)
-        assert_lxw(worksheet_set_column(sheet, i, i, 20, date));
+        assert_lxw(worksheet_set_column(sheet, i, i, 11, date));
       if(coltypes[i] == COL_POSIXCT)
-        assert_lxw(worksheet_set_column(sheet, i, i, 20, datetime));
+        assert_lxw(worksheet_set_column(sheet, i, i, 21, datetime));
     }
 
     // Need to iterate by row first for performance