Add some discussion about KDM timezones.

diff --git a/doc/design/kdm_timezones.tex b/doc/design/kdm_timezones.tex
new file mode 100644 (file)
index 0000000..bd45691
--- /dev/null
+++ b/doc/design/kdm_timezones.tex
@@ -0,0 +1,35 @@
+\documentclass{article}
+\title{KDM timezones}
+\author{}
+\date{}
+\begin{document}
+\maketitle
+
+KDMs have their valid-from and to times expressed as a local time, with a timezone that tells you the offset from UTC.
+
+There have been various different UXs in DCP-o-matic to specify the timing and time zone for KDMs, and some discussion
+about the best way.
+
+Initially cinemas in DCP-o-matic had timezones.  Then the KDM UX would get local time, and it would be written to the
+KMDs with the appropriate timezone for the cinema.
+
+This was changed in #2300 to remove timezone from the cinema and instead the KDM UX gets a local time with a time zone
+(e.g. 14:00 UTC+2) then the KDMs are written in UTC (i.e.\ 12:00) with the -00:00 timezone (meaning `unknown').
+
+The reason for this was that cinema timezones change with DST etc., people don't want to change them all the time, and
+we don't want to try managing DSTs automatically.
+
+Then people said that this was no good, as DST is not a concern (KDMs shouldn't be issued so tightly that they could
+be messed up by DST errors) but timezones are definitely a concern as they can be up to 12 hours.
+
+Then in #2473 it was changed so that cinemas once again have timezone but the KDM timezone is set by the KDM UX.
+The KDM UX timezone is prefilled with the cinema timezone, \emph{but only if that timezone is unique}.  If multiple
+cinemas are selected the default KDM UX timezone is UTC.
+
+Then Bert says this is no good because he makes KDMs for two different cinemas, one at UTC+9 and one at UTC+11.
+The cinemas look at the times and see the UTC (-00:00) times and think they are wrong.
+
+Unfortunately I can't now remember who had the problem with DST, as it seems it was probably pretty close to correct
+in the first place.
+
+\end{document}