SF patch 876130:  add C API to datetime module, from Anthony Tuininga.
The LaTeX is untested (well, so is the new API, for that matter).
Note that I also changed NULL to get spelled consistently in concrete.tex.
If that was a wrong thing to do, Fred should yell at me.
diff --git a/Misc/ACKS b/Misc/ACKS
index 3347c6f..4465970 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -569,6 +569,7 @@
 Laurence Tratt
 John Tromp
 Jason Trowbridge
+Anthony Tuininga
 Stephen Turner
 Bill Tutt
 Doobee R. Tzeck