diff --git a/build/Makefile.asciidoc.inc b/build/Makefile.asciidoc.inc
index 1a77419..7def592 100644
--- a/build/Makefile.asciidoc.inc
+++ b/build/Makefile.asciidoc.inc
@@ -1,7 +1,7 @@
 BUILDDIR = $(TOPDIR)/build
 
 GIT_VERSION := $(shell git describe --abbrev=4 --dirty --always --tags)
-GIT_DATE := $(shell date -d @`git log -n 1 "--pretty=%at" ../.` "+%Y-%b-%e")
+GIT_DATE := $(shell $(TOPDIR)/build/unix-time-to-fmt.py `git log -n 1 "--pretty=%at" ../.`)
 
 # prepend the document name with the version numbe suffix
 #DOCS_VER = $(foreach P, $(ASCIIDOCS), $(P)-v$(shell xmllint --recover --xpath "//revnumber[position()=last()]/text()" $(P)-docinfo.xml 2>/dev/null))
diff --git a/build/unix-time-to-fmt.py b/build/unix-time-to-fmt.py
new file mode 100755
index 0000000..d081797
--- /dev/null
+++ b/build/unix-time-to-fmt.py
@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+"""
+Usage:
+
+  unix-time-to-fmt.py 1234567 [%Y-%m-%d[...]]
+
+Convert unix timestamp to a string of the given format in UTC, according to
+  https://docs.python.org/2/library/time.html
+Default is '%Y-%b-%d' --> 2016-Jan-01
+"""
+
+import sys, time
+
+fmt = '%Y-%b-%d'
+if len(sys.argv) > 2:
+  fmt = sys.argv[2]
+
+print(time.strftime(fmt, time.gmtime(float(sys.argv[1]))))