[Mono-dev] why does DateTime.Now.IsDaylightSavingTime() returns false when it should be true.

[Alistair Bush]

Ok so firstly this is like the MOST C ive ever written in my life..
and it looks ugly and ive only vagely checked that it doesn't break
the northern hemisphere.

But isn't this a better patch of the method?
(https://github.com/alistair/mono/commit/6912202aab5a424e98bc44d7b988c2791f9cccc1)

Any help turning this into an acceptable pull request would be really
appreciated.

diff --git a/mono/metadata/icall.c b/mono/metadata/icall.c
index 618e4da..7f47624 100644
--- a/mono/metadata/icall.c
+++ b/mono/metadata/icall.c
@@ -5930,10 +5930,12 @@
ves_icall_System_CurrentSystemTimeZone_GetTimeZoneData (guint32 year,
MonoArray
  struct tm start, tt;
  time_t t;

- long int gmtoff;
- int is_daylight = 0, day;
+ long int gmtoff, gmtoff_st, gmtoff_ds;
+ int day, transitioned;
  char tzone [64];

+ gmtoff_st = gmtoff_ds = transitioned = 0;
+
  MONO_ARCH_SAVE_REGS;

  MONO_CHECK_ARG_NULL (data);
@@ -5974,8 +5976,10 @@
ves_icall_System_CurrentSystemTimeZone_GetTimeZoneData (guint32 year,
MonoArray
  t += 3600*24;
  tt = *localtime (&t);

+        long int gmtoff_after = gmt_offset(&tt, t);
+
  /* Daylight saving starts or ends here. */
- if (gmt_offset (&tt, t) != gmtoff) {
+ if (gmtoff_after != gmtoff) {
  struct tm tt1;
  time_t t1;

@@ -5995,36 +5999,37 @@
ves_icall_System_CurrentSystemTimeZone_GetTimeZoneData (guint32 year,
MonoArray
  strftime (tzone, sizeof (tzone), "%Z", &tt);

  /* Write data, if we're already in daylight saving, we're done. */
- if (is_daylight) {
- mono_array_setref ((*names), 0, mono_string_new (domain, tzone));
- mono_array_set ((*data), gint64, 1, ((gint64)t1 + EPOCH_ADJUST) * 10000000L);
- return 1;
+ if (tt.tm_isdst) {
+ mono_array_setref ((*names), 1, mono_string_new (domain, tzone));
+ mono_array_set ((*data), gint64, 0, ((gint64)t1 + EPOCH_ADJUST) * 10000000L);
+ if (gmtoff_ds == 0) {
+ gmtoff_st = gmtoff;
+ gmtoff_ds = gmtoff_after;
+ }
+ transitioned++;
  } else {
- struct tm end;
  time_t te;
+ te = mktime (&tt);

- memset (&end, 0, sizeof (end));
- end.tm_year = year-1900 + 1;
- end.tm_mday = 1;
-
- te = mktime (&end);
-
- mono_array_setref ((*names), 1, mono_string_new (domain, tzone));
- mono_array_set ((*data), gint64, 0, ((gint64)t1 + EPOCH_ADJUST) * 10000000L);
  mono_array_setref ((*names), 0, mono_string_new (domain, tzone));
  mono_array_set ((*data), gint64, 1, ((gint64)te + EPOCH_ADJUST) * 10000000L);
- is_daylight = 1;
+ if (gmtoff_ds == 0) {
+ gmtoff_st = gmtoff_after;
+ gmtoff_ds = gmtoff;
+ }
+ transitioned++;
  }

  /* This is only set once when we enter daylight saving. */
- mono_array_set ((*data), gint64, 2, (gint64)gmtoff * 10000000L);
- mono_array_set ((*data), gint64, 3, (gint64)(gmt_offset (&tt, t) -
gmtoff) * 10000000L);
-
+ if (tt1.tm_isdst) {
+ mono_array_set ((*data), gint64, 2, (gint64)gmtoff_st * 10000000L);
+ mono_array_set ((*data), gint64, 3, (gint64)(gmtoff_ds - gmtoff_st)
* 10000000L);
+ }
  gmtoff = gmt_offset (&tt, t);
  }
  }

- if (!is_daylight) {
+ if (transitioned < 2) {
  strftime (tzone, sizeof (tzone), "%Z", &tt);
  mono_array_setref ((*names), 0, mono_string_new (domain, tzone));
  mono_array_setref ((*names), 1, mono_string_new (domain, tzone));

On Tue, Oct 29, 2013 at 9:13 AM, Alistair Bush <alistair.bush at gmail.com> wrote:
> Well that certainly sucks.
>
>
> On Tue, Oct 29, 2013 at 3:03 AM, Robert Jordan <robertj at gmx.net> wrote:
>>
>> On 28.10.2013 07:35, Alistair Bush wrote:
>>>
>>> I am trying to figure out why exactly running
>>> DateTime.Now.IsDaylightSavingTIme() returns false.
>>> I live in the Auckland/Pacific timezone and pretty much everywhere I
>>> look it confirms that yes it is daylight saving time.
>>
>>
>> Unfortunately, I don't remember the details, but I'm pretty
>> sure that ves_icall_System_CurrentSystemTimeZone_GetTimeZoneData
>> has a bug w/ respect to the Southern Hemisphere.
>>
>> The code assumes that a year always begins outside a DST zone,
>> and this is obviously incorrect for the Southern Hemisphere.
>>
>> To fix this, the local var "is_daylight" must be properly
>> initialized. Currently, it's always 0 at start, but it must
>> be initialized with 1 when January, 1st is inside a DST
>> zone.
>>
>> Maybe this:
>>
>>         is_daylight = start.tm_isdst > 0;
>>
>> just before
>>
>>         /* For each day of the year, calculate the tm_gmtoff. */
>>         for (day = 0; day < 365; day++) {
>>
>> Robert
>>
>>
>> _______________________________________________
>> Mono-devel-list mailing list
>> Mono-devel-list at lists.ximian.com
>> http://lists.ximian.com/mailman/listinfo/mono-devel-list
>
>