From 80bd3766dd8f1729187ed9c8e0ad29f67cea0886 Mon Sep 17 00:00:00 2001
From: Bernhard M. Wiedemann
Date: Mon, 2 Dec 2019 11:43:18 +0100
Subject: Allow to override build date with SOURCE_DATE_EPOCH

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Fixes #11037
---
 INSTALL                                                         | 4 ++++
 configure.ml                                                    | 8 +++++++-
 doc/changelog/11-infrastructure-and-dependencies/11227-date.rst | 5 +++++
 3 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 doc/changelog/11-infrastructure-and-dependencies/11227-date.rst

diff --git a/INSTALL b/INSTALL
index d9efd55b95..31758203fe 100644
--- a/INSTALL
+++ b/INSTALL
@@ -152,6 +152,10 @@ INSTALLATION PROCEDURE IN DETAILS (NORMAL USERS).
 
         c.f. https://caml.inria.fr/mantis/view.php?id=7630
 
+   If you want your build to be reproducible, ensure that the
+   SOURCE_DATE_EPOCH environment variable is set as documented in
+   https://reproducible-builds.org/specs/source-date-epoch/
+
 4- Still in the root directory, do
 
 	make
diff --git a/configure.ml b/configure.ml
index 88bc3f912b..89d9ed9d2a 100644
--- a/configure.ml
+++ b/configure.ml
@@ -194,6 +194,12 @@ let which prog =
 let program_in_path prog =
   try let _ = which prog in true with Not_found -> false
 
+let build_date =
+  try
+    float_of_string (Sys.getenv "SOURCE_DATE_EPOCH")
+  with
+    Not_found -> Unix.time ()
+
 (** * Date *)
 
 (** The short one is displayed when starting coqtop,
@@ -204,7 +210,7 @@ let months =
     "July";"August";"September";"October";"November";"December" |]
 
 let get_date () =
-  let now = Unix.localtime (Unix.time ()) in
+  let now = Unix.gmtime build_date in
   let year = 1900+now.Unix.tm_year in
   let month = months.(now.Unix.tm_mon) in
   sprintf "%s %d" month year,
diff --git a/doc/changelog/11-infrastructure-and-dependencies/11227-date.rst b/doc/changelog/11-infrastructure-and-dependencies/11227-date.rst
new file mode 100644
index 0000000000..5c08e2b0ea
--- /dev/null
+++ b/doc/changelog/11-infrastructure-and-dependencies/11227-date.rst
@@ -0,0 +1,5 @@
+- **Added:**
+  Build date can now be overriden by setting the `SOURCE_DATE_EPOCH`
+  environment variable
+  (`#11227 <https://github.com/coq/coq/pull/11227>`_,
+  by Bernhard M. Wiedemann).
-- 
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