diff --git a/guix/build/gnu-build-system.scm b/guix/build/gnu-build-system.scm
index e168b6c7e9..2abaa6efdc 100644
--- a/guix/build/gnu-build-system.scm
+++ b/guix/build/gnu-build-system.scm
@@ -39,6 +39,13 @@ (define-module (guix build gnu-build-system)
 ;;
 ;; Code:
 
+(define* (set-SOURCE-DATE-EPOCH #:rest _)
+  "Set the 'SOURCE_DATE_EPOCH' environment variable.  This is used by tools
+that incorporate timestamps as a way to tell them to use a fixed timestamp.
+See https://reproducible-builds.org/specs/source-date-epoch/."
+  (setenv "SOURCE_DATE_EPOCH" "1")
+  #t)
+
 (define (first-subdirectory dir)
   "Return the path of the first sub-directory of DIR."
   (file-system-fold (lambda (path stat result)
@@ -549,7 +556,7 @@ (define %standard-phases
   ;; Standard build phases, as a list of symbol/procedure pairs.
   (let-syntax ((phases (syntax-rules ()
                          ((_ p ...) `((p . ,p) ...)))))
-    (phases set-paths install-locale unpack
+    (phases set-SOURCE-DATE-EPOCH set-paths install-locale unpack
             patch-usr-bin-file
             patch-source-shebangs configure patch-generated-file-shebangs
             build check install
@@ -577,9 +584,6 @@ (define (elapsed-time end start)
   ;; Encoding/decoding errors shouldn't be silent.
   (fluid-set! %default-port-conversion-strategy 'error)
 
-  ;; Avoid non-determinism related to generated timestamps.
-  (setenv "SOURCE_DATE_EPOCH" "1")
-
   ;; The trick is to #:allow-other-keys everywhere, so that each procedure in
   ;; PHASES can pick the keyword arguments it's interested in.
   (every (match-lambda
diff --git a/guix/build/python-build-system.scm b/guix/build/python-build-system.scm
index 8025b7fec6..9109fb4ac7 100644
--- a/guix/build/python-build-system.scm
+++ b/guix/build/python-build-system.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2015 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
@@ -136,18 +136,11 @@ (define* (ensure-no-mtimes-pre-1980 #:rest _)
                #t))
     #t))
 
-(define* (set-SOURCE-DATE-EPOCH #:rest _)
-  "Set the 'SOURCE_DATE_EPOCH' environment variable."
-  ;; Use zero as the timestamp in .pyc files so that builds are deterministic.
-  ;; TODO: Remove it when this variable is set in GNU:%STANDARD-PHASES.
-  (setenv "SOURCE_DATE_EPOCH" "1"))
-
 (define %standard-phases
   ;; 'configure' and 'build' phases are not needed.  Everything is done during
   ;; 'install'.
   (modify-phases gnu:%standard-phases
     (add-after 'unpack 'ensure-no-mtimes-pre-1980 ensure-no-mtimes-pre-1980)
-    (add-after 'unpack 'set-SOURCE-DATE-EPOCH set-SOURCE-DATE-EPOCH)
     (delete 'configure)
     (replace 'install install)
     (replace 'check check)