diff --git a/Makefile.inc1 b/Makefile.inc1
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -35,7 +35,10 @@
 #	KERNEL_FLAGS= additional flags to pass to make(1) during buildkernel
 #	SUBDIR_OVERRIDE="list of dirs" to build rather than everything.
 #	    All libraries and includes, and some build tools will still build.
-
+#	REPRODUCIBLE_EPOCH="timestamp" to use instead of gmtime(3) for all
+#		timestamps in the build.  This is used to facilitate reproducible
+#		build artifacts from the same source tree, with build tools like
+#		thermite.
 #
 # The intended user-driven targets are:
 # buildworld  - rebuild *everything*, including glue to help do upgrades
@@ -573,7 +576,7 @@
 
 _PKG_REVISION=	${_REVISION}
 _STRTIMENOW=	%Y%m%d%H%M%S
-_TIMENOW=	${_STRTIMENOW:gmtime}
+_TIMENOW=	${_STRTIMENOW:gmtime=${REPRODUCIBLE_EPOCH:U0}}
 BRANCH_EXT=
 .if ${_BRANCH:MCURRENT*} || ${_BRANCH:MSTABLE*} || ${_BRANCH:MPRERELEASE*}
 _PKG_REVISION:= ${MAJOR_REVISION}
@@ -602,7 +605,7 @@
 SOURCE_DATE_EPOCH!=	${GIT_CMD} -C ${SRCDIR} show -s --format=%ct HEAD
 .else
 TIMEEPOCHNOW=		%s
-SOURCE_DATE_EPOCH=	${TIMEEPOCHNOW:gmtime}
+SOURCE_DATE_EPOCH=	${TIMEEPOCHNOW:gmtime=${REPRODUCIBLE_EPOCH:U0}}
 .endif
 .else
 SOURCE_DATE_EPOCH=	${PKG_TIMESTAMP}