Index: sys/boot/efi/boot1/Makefile
===================================================================
--- sys/boot/efi/boot1/Makefile
+++ sys/boot/efi/boot1/Makefile
@@ -94,6 +94,9 @@
 		echo "Undefined symbols in ${.ALLSRC}"; \
 		exit 1; \
 	fi
+	# Arbitrarily set the PE/COFF header timestamps to 1 Jan 2016 00:00:00
+	# for build reproducibility.
+	SOURCE_DATE_EPOCH=1451606400 \
 	${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
@@ -140,6 +140,9 @@
 		echo "Undefined symbols in ${.ALLSRC}"; \
 		exit 1; \
 	fi
+	# Arbitrarily set the PE/COFF header timestamps to 1 Jan 2016 00:00:00
+	# for build reproducibility.
+	SOURCE_DATE_EPOCH=1451606400 \
 	${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 \