cairo's PNG output includes a tIME chunk.  cairo-png.c line 174-175:
-------------------------------
  png_convert_from_time_t (&pt, time (NULL));
  png_set_tIME (png, info, &pt);
-------------------------------

However,
1) this makes the resultant PNG files (very slightly) bigger.
2) this is unnecessary - it does not affect what an image viewer (e.g.
eog) displays, since tIME is an "ancillary [PNG] chunk".
3) this means that every time I run my code, the output changes, even
if the code itself does not.  This is my biggest concern, because it
means that I can't use vanilla cairo to write unit tests that looks
like this:
-------------------------------
string golden_output = load_file("golden.png");
string actual_output = use_cairo_to_draw_stuff_to_png(...);
assert_equal(golden_output, actual_output);
-------------------------------

It also looks like this has come up before - search for "XXX: Setting
the time is interfereing with the image comparison" or just read the
Log Message at the top of
http://lists.freedesktop.org/archives/cairo-commit/2004-May/000886.html

Now, hooray for open source, I can just patch my own copy of cairo to
remove these two lines.  But should this go upstream??  Is there a
reason why tIMEs are useful/required, or should we just nuke it out
for the 1.4 series?

As raised in the mailing list [1], and re-posted to bugzilla so that it can be
tracked, even if the conclusion is a simple WONTFIX.  :-)

[1] http://lists.freedesktop.org/archives/cairo/2006-August/007690.html