diff --git a/examples/wch/ch32v307-make-baremetal-builtin/Makefile b/examples/wch/ch32v307-make-baremetal-builtin/Makefile
index 0c0a0989..fc1aee48 100644
--- a/examples/wch/ch32v307-make-baremetal-builtin/Makefile
+++ b/examples/wch/ch32v307-make-baremetal-builtin/Makefile
@@ -35,7 +35,7 @@ flash: firmware.bin
 #	@echo; echo "IMPORTANT: configure device to 96k RAM / 224k Flash"
 
 zip: firmware.bin
-	zip -r ch32v307.modbus.firmware.`+%Y-%m-%d`.zip  firmware.*
+	zip -r ch32v307.modbus.firmware.`date +%Y-%m-%d`.zip  firmware.*
 
 clean:
 	$(RM) firmware.* *.zip