[Ur] Better date & time support in Ur/Web + PostgreSQL

[Simon Van Casteren]

I've been building www.classy.school for some time with Ur/Web now. It's an
application for music schools and a lot of it revolves around schedules:
Who has lessons at what time, which teacher, which room, etc etc.

In Ur/Web's "standard library" there are 2 types to represent dates / times:

   - Basis.time (Corresponds to unix epoch milliseconds IIRC). Contains
   both date and time
   - Datetime.t (Corresponds to a C struct IIRC). A bit more structure than
   Basis.time

I don't work with these two types at all. I defined two other types:

   - calendardate. This is actually a type synonym for Basis.time, but only
   because it makes it possible to serialize this to sql values. All
   operations on this type only change the date part, so year - month - day.
   It contains no timezone info.
   - clocktime: { Hour: int, Minute: int}. (I don't need seconds, but it
   wouldn't hurt to add it as well). I have to serialize / deserialize this
   whenever it goes into the DB, very annoying.

I've found this to be a much easier representation to work with for my
domain. Example: When you enroll with a teacher for some private lessons,
you often do it for x (eg: 10) lessons on a certain weekday on a certain
time. This time I have in my datamodel as a clocktime. The actual
"timestamps" of every lesson are seperate. Another benefit: Comparing
calendardates is much easier than comparing Basis.time / Datetime.t.

Anyway, I've been thinking for some time to propose to upstream all of this
/ some of this into the standard library, if there is any interest for it.
With that I'd also serialize them into the correct PostgreSQL types
(calendardate -> date, clocktime -> time without timezone). Afterwards, I
want to look into adding support for some SQL operators on these,
especially adding a clocktime to a date (which then becomes a PostgreSQL
timestamp without timezone, not sure yet how to model this in the type
system). Being able to do this in SQL would be huge for my application.

So long story short, I'd mainly like to know if adding this stuff to the
standard library would be welcomed. If not, I'll keep all this in my
personal repo and put the SQL stuff in my urweb fork, but I thought I'd ask
:).

Simon
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