QDateTime: How does one distinguish the "double hour" during DST switch?

Asperamanca

According to docs, QDateTime supports daylight saving time. But how to I distinguish the twin time periods that happen when the clock is turned back during a DST switch? For example, before reaching 3am, the clock is turned back to 2am. If QDateTime says it's 2:39am, are we talking about the first or second such period?
Also, the toString() method seems to lack a way to express this.

m.sue

Hi,

I would guess that the function isDaylightTime() returns true during the first and false during the second such hour.

-Michael.

Asperamanca

@m.sue said in QDateTime: How does one distinguish the "double hour" during DST switch?:

Hi,

I would guess that the function isDaylightTime() returns true during the first and false during second such hour.

-Michael.

True. That would still make it very cumbersome to "mark" the second of a duplicate time span, because I would need to understand the specific rules of the DST switch to know whether this is a time period that I need to mark at all.

Here is an example of what I am looking for.

UTC      DST   DESIRED OUTPUT
01:00:00 true  02:00:00
01:30:00 true  02:30:00
02:00:00 false 02b00:00
02:30:00 false 02b30:00
03:00:00 false 03:00:00

The details of the DST switch should best remain buried within Qt - as a user of the class, I would rather not know the details whether my timezone has DST at all, when and how often the switch occurs, and with what time difference.

SGaist

Hi,

In that case, shouldn't you use a date time format that includes time zone information ? You'll then have the offset applied with DST and without.

Asperamanca

@SGaist said in QDateTime: How does one distinguish the "double hour" during DST switch?:

Hi,

In that case, shouldn't you use a date time format that includes time zone information ? You'll then have the offset applied with DST and without.

I think you refer to the ISO 8601 format (Qt::ISODate). That format gives you time zone information, but it doesn't tell you (at one glance) whether you are in a double hour, either. As a user, you again have to understand the time zone specification, and I'd like to avoid that.
Also, it is rather long.

hskoglund

Hi, I think @SGaist meant the vanilla QDateTime, anyway, what you can try:
#include "qdatetime.h"
and

if (!QDateTime::currentDateTime().isDaylightTime() && QDateTime::currentDateTime().addSecs(-3600).isDaylightTime())
    // inside the double hour

Asperamanca

@hskoglund said in QDateTime: How does one distinguish the "double hour" during DST switch?:

Hi, I think @SGaist meant the vanilla QDateTime, anyway, what you can try:
#include "qdatetime.h"
and
if (!QDateTime::currentDateTime().isDaylightTime() && QDateTime::currentDateTime().addSecs(-3600).isDaylightTime())
    // inside the double hour

Good point. That could work. Thanks!