diff --git a/arch/um/Makefile b/arch/um/Makefile
index 322972fd064eac3d45f363c4dd87f56da500d406..45435ff589c17e8252008431e3ee659dedc08ed8 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -67,7 +67,8 @@ USER_CFLAGS := $(patsubst -D__KERNEL__,,$(USER_CFLAGS)) $(ARCH_INCLUDE) \
 # in CFLAGS.  Otherwise, it would cause ld to complain about the two different
 # errnos.
 
-CFLAGS += -Derrno=kernel_errno -Dsigprocmask=kernel_sigprocmask
+CFLAGS += -Derrno=kernel_errno -Dsigprocmask=kernel_sigprocmask \
+	-Dmktime=kernel_mktime
 CFLAGS += $(call cc-option,-fno-unit-at-a-time,)
 
 include $(srctree)/$(ARCH_DIR)/Makefile-$(SUBARCH)