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scribblings/gregor-utils.scrbl

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+#lang scribble/manual
+
+@(require
+   scribble/example
+   scribble/core
+   (for-label racket/base
+              racket/string
+              ))
+
+(define myeval
+   (make-base-eval '(require gregor gregor-utils)))
+
+@title[#:tag "gregor-utils"]{Some utility functions around the gregor date module}
+
+@author[@author+email["Hans Dijkema" "hans@dijkewijk.nl"]]
+
+@defmodule[gregor-utils]{This module provides some utility functions around the gregor date module}
+
+@section{Utility functions}
+
+@defproc[(date->moment [dt (or/c date? date*?)]) moment?]{
+  Converts a racket @racket{date} or @racket{date*} structure to a gregor @racket{moment}.
+  When it converts, it will assumes two kinds of dates.
+
+ @itemlist[
+     @item{The first being a standard racket date structure with a time zone offset.}
+     @item{The second for a date* structure, that will hold a named local timezone or 'UTC'.}
+     ]
+
+ This function will check if the date* structure is UTC time or local timezone. If it is
+ local timezone, it will use the function @racket{(system-tzid)} of tzinfo to determine the
+ local timezone id, that can be used to create a new gregor @racket{moment}.
+
+ As date* will be either localtime or UTC, we're working with what we got. We can recognize UTC, but the
+ timezone name will be something OS specific, e.g. on a dutch windows system: "West-Europa (zomertijd)".
+ This is reported e.g. in the windows powershell command:
+
+@#reader scribble/comment-reader
+[racketblock
+> get-timezone:
+
+  Id                         : W. Europe Standard Time
+  DisplayName                : (UTC+01:00) Amsterdam, Berlijn, Bern, Rome, Stockholm, Wenen
+  StandardName               : West-Europa (standaardtijd)
+  DaylightName               : West-Europa (zomertijd)
+  BaseUtcOffset              : 01:00:00
+  SupportsDaylightSavingTime : True
+]
+
+But, as the racket standard date functions only work with UTC and localtime, we can request the current timezone
+tzinfo's @racket{(system-tzid)} for localtime and use UTC for univeral time.
+}
+
+@defproc[(moment->date [m moment?]) date*?]{
+  Converts a gregor @racket{moment} to a standard racket @racket{date*}.
+}
+
+@defproc[(racket-date? [d any/c?]) boolean?]{
+  Returns #t, if d is of racket's standard type @racket{date} or racket{date*}.
+}