diff --git a/Makefile b/Makefile
index bfdcecd..cf8aa76 100644
--- a/Makefile
+++ b/Makefile
@@ -3,15 +3,22 @@ default: all
 
 # Overridable defaults
 
+export SOURCE_DATE_EPOCH ?= $(shell $(PROJECT_ROOT)/build/get_source_date_epoch.sh)
+export BUILDTIME ?= $(shell date -u -d '@$(SOURCE_DATE_EPOCH)' --rfc-3339 ns 2>/dev/null | sed -e 's/ /T/')
+
 VERBOSE ?=
+
 PROJECT_NAME ?= webfs
 PROJECT_ROOT ?= .
-OUT ?= $(PROJECT_ROOT)/.build
 VERSION ?= $(shell cat $(PROJECT_ROOT)/VERSION)
+OUT ?= $(PROJECT_ROOT)/.build
+
 PARALLELMFLAGS ?= -j$(shell nproc)
 UID ?= $(shell id -u)
+
 DOCKER ?= docker
 DOCKER_BUILDKIT ?= 
+
 CONTAINER_USER ?= $(UID)
 CONTAINER_GROUP ?= $(shell id -g)
 CONTAINER_WORKSPACE ?= /workspace
@@ -56,6 +63,7 @@ DOCKER_RUNFLAGS += --init
 DOCKER_RUNFLAGS += --user $(CONTAINER_USER):$(CONTAINER_GROUP)
 DOCKER_RUNFLAGS += --device /dev/fuse --cap-add SYS_ADMIN --security-opt apparmor:unconfined
 DOCKER_RUNFLAGS += --env SOURCE_DATE_EPOCH
+DOCKER_RUNFLAGS += --env BUILDTIME
 
 DOCKER_BUILDARGS += CODENAME=$(CODENAME)
 DOCKER_BUILDARGS += PARALLELMFLAGS=$(PARALLELMFLAGS)
@@ -148,7 +156,7 @@ $(OUT)/docker/qemu-arm-static-$(QEMU_VERSION):
 	  && curl -fsSL -o $@ $(URL) \
 	  && chmod +x $@ 
 
-$(OUT)/% : $(PROJECT_ROOT)/% | $(OUT_DIRS)
+$(OUT)/docker/% : $(PROJECT_ROOT)/build/% | $(OUT_DIRS)
 	cp $< $@
 
 $(OUT)/docker/%: $(OUT)/docker/%.dockerfile $(EXTRACT_TARGETS) $(PROJECT_ROOT)/Makefile | $(OUT_DIRS)
diff --git a/docker/amd64-ubuntu-builder.dockerfile b/build/amd64-ubuntu-builder.dockerfile
similarity index 100%
rename from docker/amd64-ubuntu-builder.dockerfile
rename to build/amd64-ubuntu-builder.dockerfile
diff --git a/docker/arm32v7-ubuntu-builder.dockerfile b/build/arm32v7-ubuntu-builder.dockerfile
similarity index 100%
rename from docker/arm32v7-ubuntu-builder.dockerfile
rename to build/arm32v7-ubuntu-builder.dockerfile
diff --git a/build/get_source_date_epoch.sh b/build/get_source_date_epoch.sh
new file mode 100755
index 0000000..a8bb69a
--- /dev/null
+++ b/build/get_source_date_epoch.sh
@@ -0,0 +1,26 @@
+#!/bin/sh
+
+export LANG=C
+export LC_ALL=C
+
+[ -n "$TOPDIR" ] && cd "$TOPDIR"
+
+try_git() {
+	[ -e .git ] || return 1
+	SOURCE_DATE_EPOCH="$(git log -1 --format=format:%ct)"
+	[ -n "$SOURCE_DATE_EPOCH" ]
+}
+
+try_hg() {
+	[ -d .hg ] || return 1
+	SOURCE_DATE_EPOCH="$(hg log --template '{date}' -l 1 | cut -d. -f1)"
+	[ -n "$SOURCE_DATE_EPOCH" ]
+}
+
+try_mtime() {
+	perl -e 'print((stat $ARGV[0])[9])' "$0"
+	[ -n "$SOURCE_DATE_EPOCH" ]
+}
+
+try_git || try_hg || try_mtime || SOURCE_DATE_EPOCH=""
+echo "$SOURCE_DATE_EPOCH"