for b in $(git tag | grep -- '-beta'); do printf "%s: %s\n" "$b" "$(git show $b | grep -A3 ^commit | grep ^Date:)" ; done

gistfile1.txt

15.09-beta: Date:   Wed Aug 26 22:52:10 2015 +0200
16.03-beta: Date:   Sun Feb 28 22:30:51 2016 +0000
16.09-beta: Date:   Thu Sep 1 21:10:20 2016 +0200
17.03-beta: Date:   Mon Feb 27 20:12:01 2017 +0100
17.09-beta: Date:   Wed Aug 30 22:51:32 2017 +0200
18.03-beta: Date:   Mon Mar 5 18:03:02 2018 +0000

raw data.txt

$ for b in $(git tag | grep -- '-beta'); do git show $b | cat && echo "==========" ; done | xclip

tag 15.09-beta
Tagger: Domen Kožar <domen@dev.si>
Date:   Sun Oct 4 15:30:47 2015 +0200

Release 15.09-beta

commit 977731f6c2227bc48ff2f9f628a30e3e38e09b4b
Author: Cillian de Róiste <cillian.deroiste@gmail.com>
Date:   Wed Aug 26 22:52:10 2015 +0200

    synthv1: update from 0.6.3 to 0.7.1

diff --git a/pkgs/applications/audio/synthv1/default.nix b/pkgs/applications/audio/synthv1/default.nix
index 4a126f8774f..43003782f2f 100644
--- a/pkgs/applications/audio/synthv1/default.nix
+++ b/pkgs/applications/audio/synthv1/default.nix
@@ -2,11 +2,11 @@
 
 stdenv.mkDerivation rec {
   name = "synthv1-${version}";
-  version = "0.6.3";
+  version = "0.7.1";
 
   src = fetchurl {
     url = "mirror://sourceforge/synthv1/${name}.tar.gz";
-    sha256 = "19zyvrvnmi7ahwg121vl2q17j9y8ln6lvpj5wxxcwif5538q75iw";
+    sha256 = "0asjhz0xj1kwysvsj9q54r8j8fy7cnr408ygfpdhg7yn24rv67hh";
   };
 
   buildInputs = [ qt4 libjack2 lv2 ];
==========
tag 16.03-beta
Tagger: Vladimír Čunát <vcunat@gmail.com>
Date:   Tue Mar 1 11:21:33 2016 +0100

Release 16.03-beta

commit 74f22ff827d7c91fe26a62c69a53556a62ecc70c
Author: Domen Kožar <domen@dev.si>
Date:   Sun Feb 28 22:30:51 2016 +0000

    prepare for 16.03

diff --git a/nixos/modules/misc/version.nix b/nixos/modules/misc/version.nix
index 18f270cd531..c062e81c69b 100644
--- a/nixos/modules/misc/version.nix
+++ b/nixos/modules/misc/version.nix
@@ -89,7 +89,7 @@ in
     defaultChannel = mkOption {
       internal = true;
       type = types.str;
-      default = https://nixos.org/channels/nixos-unstable;
+      default = https://nixos.org/channels/nixos-16.03;
       description = "Default NixOS channel to which the root user is subscribed.";
     };
 
diff --git a/nixos/release.nix b/nixos/release.nix
index cfe152cc163..a0ed18c3678 100644
--- a/nixos/release.nix
+++ b/nixos/release.nix
@@ -9,7 +9,7 @@ let
 
   version = builtins.readFile ../.version;
   versionSuffix =
-    (if stableBranch then "." else "pre") + "${toString nixpkgs.revCount}.${nixpkgs.shortRev}";
+    (if stableBranch then "." else "pre") + "${toString (nixpkgs.revCount - 77900)}.${nixpkgs.shortRev}";
 
   forAllSystems = genAttrs supportedSystems;
 
==========
tag 16.09-beta
Tagger: Domen Kožar <domen@dev.si>
Date:   Thu Sep 1 21:22:53 2016 +0200

Release 16.09-beta

commit 7c71a897dd0c641e047d6e713dca85764b577c4e
Author: Domen Kožar <domen@dev.si>
Date:   Thu Sep 1 21:10:20 2016 +0200

    php: 5.6 -> 7.0
    
    I'll probably regret this in testing period, but let's bump php
    since 5.6 support expires in December.

diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 9c53f2208a1..9f6870697d1 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -5851,9 +5851,9 @@ in
 
   inherit (callPackages ../development/interpreters/perl {}) perl perl520 perl522;
 
-  php = php56;
+  php = php70;
 
-  phpPackages = php56Packages;
+  phpPackages = php70Packages;
 
   php56Packages = recurseIntoAttrs (callPackage ./php-packages.nix {
     php = php56;
==========
tag 17.03-beta
Tagger: Robin Gloster <mail@glob.in>
Date:   Mon Feb 27 20:16:47 2017 +0100

release 17.03-beta
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commit 6c9fb36526dffe3e62d411fc8cdaad82bb6fbfed
Author: Robin Gloster <mail@glob.in>
Date:   Mon Feb 27 20:12:01 2017 +0100

    release 17.03-beta

diff --git a/nixos/modules/misc/version.nix b/nixos/modules/misc/version.nix
index bf20e801f61..5ddd495532b 100644
--- a/nixos/modules/misc/version.nix
+++ b/nixos/modules/misc/version.nix
@@ -78,7 +78,7 @@ in
     defaultChannel = mkOption {
       internal = true;
       type = types.str;
-      default = https://nixos.org/channels/nixos-unstable;
+      default = https://nixos.org/channels/nixos-17.03;
       description = "Default NixOS channel to which the root user is subscribed.";
     };
 
diff --git a/nixos/release.nix b/nixos/release.nix
index 2bfe8eada0b..c618ea1879a 100644
--- a/nixos/release.nix
+++ b/nixos/release.nix
@@ -9,7 +9,7 @@ let
 
   version = fileContents ../.version;
   versionSuffix =
-    (if stableBranch then "." else "pre") + "${toString nixpkgs.revCount}.${nixpkgs.shortRev}";
+    (if stableBranch then "." else "beta") + "${toString (nixpkgs.revCount - 102184)}.${nixpkgs.shortRev}";
 
   forAllSystems = genAttrs supportedSystems;
 
==========
tag 17.09-beta
Tagger: Franz Pletz <fpletz@fnordicwalking.de>
Date:   Wed Aug 30 22:53:03 2017 +0200

Release 17.09-beta
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commit 6a7066eb53685438564dc50362666d0e7f150110
Author: Franz Pletz <fpletz@fnordicwalking.de>
Date:   Wed Aug 30 22:51:32 2017 +0200

    release documentation: reorder beta steps

diff --git a/nixos/doc/manual/development/releases.xml b/nixos/doc/manual/development/releases.xml
index bcc746a67b7..afcb970ed70 100755
--- a/nixos/doc/manual/development/releases.xml
+++ b/nixos/doc/manual/development/releases.xml
@@ -34,8 +34,9 @@
     <itemizedlist spacing="compact">
       <listitem>
         <para>
-          Create a new release notes file for the upcoming release + 1, in this
-          case <literal>rl-1803.xml</literal>.
+          <link xlink:href="https://github.com/NixOS/nixpkgs/issues/13559">Create
+            an issue for tracking Zero Hydra Failures progress. ZHF is an effort
+            to get build failures down to zero.</link>
         </para>
       </listitem>
       <listitem>
@@ -95,9 +96,8 @@
       </listitem>
       <listitem>
         <para>
-          <link xlink:href="https://github.com/NixOS/nixpkgs/issues/13559">Create
-            an issue for tracking Zero Hydra Failures progress. ZHF is an effort
-            to get build failures down to zero.</link>
+          Create a new release notes file for the upcoming release + 1, in this
+          case <literal>rl-1803.xml</literal>.
         </para>
       </listitem>
       <listitem>
==========
tag 18.03-beta
Tagger: Vladimír Čunát <vcunat@gmail.com>
Date:   Mon Mar 5 19:28:31 2018 +0100

Release 18.03-beta
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commit 1bc5bf4beb759e563ffc7a8a3067f10a00b45a7d
Merge: 8bcdb39a271 89d0307a391
Author: zimbatm <zimbatm@zimbatm.com>
Date:   Mon Mar 5 18:03:02 2018 +0000

    Merge pull request #36283 from aneeshusa/fix-vagrant-2.0.2
    
    Fix vagrant 2.0.2

==========