diff --git a/Makefile b/Makefile
index 4e9e16212..8724fe140 100644
--- a/Makefile
+++ b/Makefile
@@ -368,11 +368,9 @@ endif
 		mv *.AppImage ../../koreader-$(DIST)-$(MACHINE)-$(VERSION).AppImage
 
 androidupdate: all
-	# in runtime luajit-launcher's libluajit.so will be loaded
-	-rm $(INSTALL_DIR)/koreader/libs/libluajit.so
-
-        # fresh APK assets
-	rm -rfv $(ANDROID_ASSETS) $(ANDROID_LIBS_ROOT)
+	# Note: do not remove the module directory so there's no need
+	# for `mk7z.sh` to always recreate `assets.7z` from scratch.
+	rm -rfv $(ANDROID_LIBS_ROOT)
 	mkdir -p $(ANDROID_ASSETS) $(ANDROID_LIBS_ABI)
 
 	# APK version
@@ -380,6 +378,8 @@ androidupdate: all
 
 	# shared libraries are stored as raw assets
 	cp -pR $(INSTALL_DIR)/koreader/libs $(ANDROID_LAUNCHER_DIR)/assets
+	# in runtime luajit-launcher's libluajit.so will be loaded
+	rm -vf $(ANDROID_LAUNCHER_DIR)/assets/libs/libluajit.so
 
 	# binaries are stored as shared libraries to prevent W^X exception on Android 10+
 	# https://developer.android.com/about/versions/10/behavior-changes-10#execute-permission
@@ -387,31 +387,34 @@ androidupdate: all
 	echo "sdcv libsdcv.so" > $(ANDROID_ASSETS)/map.txt
 
 	# assets are compressed manually and stored inside the APK.
-	cd $(INSTALL_DIR)/koreader && 7z a -l -m0=lzma2 -mx=9 \
-		../../$(ANDROID_ASSETS)/koreader.7z * \
-		-xr!*cache$ \
-		-xr!*clipboard$ \
-		-xr!*data/dict$ \
-		-xr!*data/tessdata$ \
-		-xr!*history$ \
-		-xr!*l10n/templates$ \
-		-xr!*libs$ \
-		-xr!*ota$ \
-		-xr!*resources/fonts* \
-		-xr!*resources/icons/src* \
-		-xr!*rocks/bin$ \
-		-xr!*rocks/lib/luarocks$ \
-		-xr!*screenshots$ \
-		-xr!*share/man* \
-		-xr!*spec$ \
-		-xr!*tools$ \
-		-xr!*COPYING$ \
-		-xr!*Makefile$ \
-		-xr!*NOTES.txt$ \
-		-xr!*NOTICE$ \
-		-xr!*README.md$ \
-		-xr!*sdcv \
-		-xr'!.*'
+	cd $(INSTALL_DIR)/koreader && \
+		./tools/mk7z.sh \
+		../../$(ANDROID_ASSETS)/koreader.7z \
+		"$$(git show -s --format='%ci')" \
+		-m0=lzma2 -mx=9 \
+		-- . \
+		'-x!cache' \
+		'-x!clipboard' \
+		'-x!data/dict' \
+		'-x!data/tessdata' \
+		'-x!history' \
+		'-x!l10n/templates' \
+		'-x!libs' \
+		'-x!ota' \
+		'-x!resources/fonts*' \
+		'-x!resources/icons/src*' \
+		'-x!rocks/bin' \
+		'-x!rocks/lib/luarocks' \
+		'-x!screenshots' \
+		'-x!sdcv' \
+		'-x!spec' \
+		'-x!tools' \
+		'-xr!.*' \
+		'-xr!COPYING' \
+		'-xr!NOTES.txt' \
+		'-xr!NOTICE' \
+		'-xr!README.md' \
+		;
 
 	# make the android APK
 	$(MAKE) -C $(ANDROID_LAUNCHER_DIR) $(if $(KODEBUG), debug, release) \
diff --git a/tools/mk7z.sh b/tools/mk7z.sh
new file mode 100755
index 000000000..74093f5e0
--- /dev/null
+++ b/tools/mk7z.sh
@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+
+set -eo pipefail
+# set -x
+
+[[ $# -ge 3 ]]
+archive="$(realpath "$1")"
+epoch="$2"
+shift 2
+options=()
+for a in "$@"; do
+    shift
+    case "${a}" in
+        --) break ;;
+        -*) options+=("${a}") ;;
+        *) break ;;
+    esac
+done
+[[ $# -gt 0 ]]
+patterns=("$@")
+
+# We need to use the full path to the executable to avoid
+# a weird issue when using the p7zip project pre-built
+# binary (`Can't load './7z.dll' (7z.so)...`).
+sevenzip="$(which 7z)"
+
+# echo "archive : ${archive}"
+# echo "epoch   : ${epoch}"
+# echo "options : ${options[@]}"
+# echo "patterns: ${patterns[@]}"
+
+tmpdir="$(mktemp -d -t tmp7z.XXXXXXXXXX)"
+trap 'rm -rf "${tmpdir}"' EXIT
+
+manifest="${tmpdir}/manifest"
+
+"${sevenzip}" -l -ba h "${patterns[@]}" |
+    awk '{ if ($3) print $3, $2, $1; else print $1 }' |
+    sort >"${manifest}"
+
+# cat "${manifest}" | less
+
+if [[ -r "${archive}" ]]; then
+    if diff --brief --label 'in archive' \
+        <(
+            "${sevenzip}" -slt l "${archive}" |
+                awk '
+                    /^(\w+) = / { entry[$1] = $3; }
+                    /^CRC =/ { if ($3) print entry["Path"], entry["Size"], $3; else print entry["Path"] }
+                    ' | sort
+        ) --label 'to add' "${manifest}"; then
+        exit
+    fi
+    # There's no 7z option to overwrite the archive
+    # if it already exists (instead of updating it)…
+    rm -f "${archive}"
+fi
+
+# Extract list of paths from manifest.
+rev <"${manifest}" | cut -f3- -d' ' | rev >"${tmpdir}/paths"
+# Quick sanity check: no path outside the current directory.
+if grep '^(/|\.\./)' <"${tmpdir}/paths"; then
+    echo "^ some paths are outside the current directory!"
+    exit 1
+fi
+
+# Make a copy of everything so we can later
+# patch timestamp to ensure reproducibility.
+mkdir "${tmpdir}/contents"
+# We want to copy "empty" (with ignored files) directories.
+tar --dereference --no-recursion --create \
+    --verbatim-files-from --files-from="${tmpdir}/paths" |
+    tar --extract --directory="${tmpdir}/contents"
+
+cd "${tmpdir}/contents"
+
+# Fix timestamps.
+find . -depth -print0 | xargs -0 touch --date="${epoch}"
+
+# And create the final archive.
+"${sevenzip}" -l -mqs "${options[@]}" a "${archive}" .
+
+# vim: sw=4