libwebrtc and snowflake are not being built reproducibly (unless you build in the same month)

I already ran into this problem when building 7.0a4 but did not have time to investigate it back then: I did only a partial rebuild (not touching snowflake as this was not necessary) but boklm did a full build. The result was a different snowflake-client.

Now this happened with the build for 7.5a2 again. The diff is:

--- /dev/fd/63	2017-07-06 09:57:34.341983842 +0200
+++ /dev/fd/62	2017-07-06 09:57:34.341983842 +0200
@@ -750821,7 +750821,7 @@
 00b74e40: 6e6e 656c 3d25 642c 2025 6c75 2900 7468  nnel=%d, %lu).th
 00b74e50: 6973 203d 3d20 675f 746f 705f 6d61 6e61  is == g_top_mana
 00b74e60: 6765 7200 4368 6563 6b20 6661 696c 6564  ger.Check failed
-00b74e70: 3a20 6675 6e63 2e20 004a 756e 2030 3420  : func. .Jun 04 
+00b74e70: 3a20 6675 6e63 2e20 004a 756c 2030 3220  : func. .Jul 02 
 00b74e80: 3230 3137 2030 353a 3030 3a30 3000 4368  2017 05:00:00.Ch
 00b74e90: 6563 6b20 6661 696c 6564 3a20 6269 6e64  eck failed: bind
 00b74ea0: 5f73 7461 7465 5f2e 2000 2c3c 2a00 4368  _state_. .,<*.Ch

That seems to be a bit weird given the actual build date but boklm had the idea it could be the first Sunday of the month/week getting embedded.

And, indeed, as boklm pointed out /build/write_build_date_header.py seems to do such a thing.

I can trace the problem back to build_time.o in the libwebrtc .a file.