Index: sys/boot/efi/boot1/Makefile
===================================================================
--- sys/boot/efi/boot1/Makefile
+++ sys/boot/efi/boot1/Makefile
@@ -89,12 +89,15 @@
 EFI_TARGET=	binary
 .endif
 
+# Arbitrarily set the PE/COFF header timestamps to 1 Jan 2016 00:00:00
+# for build reproducibility.
+SOURCE_DATE_EPOCH?=1451606400
 boot1.efi: ${PROG}
 	if ${NM} ${.ALLSRC} | grep ' U '; then \
 		echo "Undefined symbols in ${.ALLSRC}"; \
 		exit 1; \
 	fi
-	SOURCE_DATE_EPOCH=1451606400 \
+	SOURCE_DATE_EPOCH=${SOURCE_DATE_EPOCH} \
 	${OBJCOPY} -j .peheader -j .text -j .sdata -j .data \
 		-j .dynamic -j .dynsym -j .rel.dyn \
 		-j .rela.dyn -j .reloc -j .eh_frame \
Index: sys/boot/efi/loader/Makefile
===================================================================
--- sys/boot/efi/loader/Makefile
+++ sys/boot/efi/loader/Makefile
@@ -135,12 +135,15 @@
 EFI_TARGET=	binary
 .endif
 
+# Arbitrarily set the PE/COFF header timestamps to 1 Jan 2016 00:00:00
+# for build reproducibility.
+SOURCE_DATE_EPOCH?=1451606400
 loader.efi: ${PROG}
 	if ${NM} ${.ALLSRC} | grep ' U '; then \
 		echo "Undefined symbols in ${.ALLSRC}"; \
 		exit 1; \
 	fi
-	SOURCE_DATE_EPOCH=1451606400 \
+	SOURCE_DATE_EPOCH=${SOURCE_DATE_EPOCH} \
 	${OBJCOPY} -j .peheader -j .text -j .sdata -j .data \
 		-j .dynamic -j .dynsym -j .rel.dyn \
 		-j .rela.dyn -j .reloc -j .eh_frame -j set_Xcommand_set \