Which one is best to use, DateTime or INT (Unix Timestamp) or anything else to store the time value?
I think INT will be better at performance and also more universal, since it can be easily converted to many timezones. (my web visitors from all around the world can see the time without confusion)
But, I'm still doubt about it.
Any suggestions?
I wouldn't use INT or TIMESTAMP to save your datetime values. There is the "Year-2038-Problem"! You can use DATETIME and save your datetimes for a long time.
With TIMESTAMP or numeric column types you can only store a range of years from 1970 to 2038. With the DATETIME type you can save dates with years from 1000 to 9999.
It is not recommended to use a numeric column type (INT) to store datetime information. MySQL (and other sytems too) provides many functions to handle datetime information. These functions are faster and more optimized than custom functions or calculations: https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html
To convert the timezone of your stored value to the client timezone you can use CONVERT_TZ. In this case you need to know the timezone of the server and the timezone of your client. To get the timezone of the server you can see some possibilites on this question.
Changing the client time zone The server interprets TIMESTAMP values
in the client’s current time zone, not its own. Clients in different
time zones should set their zone so that the server can properly
interpret TIMESTAMP values for them.
And if you want to get the time zone that a certain one you can do this:
CONVERT_TZ(#dt,'US/Central','Europe/Berlin') AS Berlin,
I wouldn't store it in int, you should check out MySQL Cookbook by Paul DuBois he covers lot's of things in it.Also there is a big portion about your quetion.
Related
I want to save a future wall-clock datetime for events in Django (I have timezone string stored separately).
I can't simply use the DateTimeField because it enforces timestamp with time zone and always saves time in current timezone. It doesn't handle DST or possible timezone changes between current date and the date of actual event.
I could use any of these options:
Pick any timezone to store timestamps and always throw this timezone away before applying actual timezone in Python.
Split timestamp to DateField and TimeField.
Store datetime as string.
Custom field that stores datetime as timestamp without time zone.
but it makes queries more difficult and seems quite weird.
Are there any better options I miss? This usecase seems quite common so I guess there is a better way to do that?
EDIT: my usecase:
Let's say my user want to book an appointment to 2019-12-20 10:00 and currently it's 2019-03-10. I know the timezone of this user (it's stored separately as string like 'US/Eastern').
If I assume that EST starts at November 3, 2019, the best I can do is to store timestamp to 2019-12-20 15:00:00+00:00 (or 2019-12-20 10:00-05:00. I don't want this because:
I have no idea if my tzdata has correct information for future datetime
Even if it currently does, I have no idea if there would be any unexpected change in US/Eastern timezone and it becomes worse when it's not US. Future DST changes are not guaranteed.
If user moves to different timezone, I'll have to recalculate every single appointment while taking care about DST.
If tzdata changes during this recalculation... let's not think about that.
I'd prefer to store future dates as naive datetime + timezone string like 'US/Eastern' and (almost) never construct tz-aware datetime for any date further than a week. Django + postgres currently forces me to use timestamp with time zone, which is great for logs and past events, but it has fixed offset (not even timezone name) so it doesn't fit for future wall clock datetimes.
For this usecase, let's say that I don't care about ambiguous times: not much users want to book at 02:00 AM.
I see a few possible solutions:
Set USE_TZ = False and TIME_ZONE = 'UTC' and use calendar times. No conversions will be done, so essentially you're just storing the calendar time and getting it back as a naive datetime. The main problem is that this setting is global, and is not a good one for many uses (e.g. auto_now).
As above, but set USE_TZ = True. As long as you express your calendar times in UTC, there won't be any untoward conversions. The problem here is that you'll be getting aware datetimes, so you'll have to take care to ignore or remove the time zone everywhere.
Use separate DATE_FIELD and TIME_FIELD. This may or may not be a good solution depending on what kind of queries you're trying to run.
Create your own field that uses timestamp without time zone. (Or perhaps it already exists?)
Note that this issue has nothing to do with past versus future. It's about wanting to use a fixed moment in time versus a calendar (or wall clock) time. The points you raised are certainly valid objections to using a point in time to represent a calendar time.
recently i'm going to connect to PostgreSQL and I need to store date/time in my object to pass to the query for insert and update some table.
but there is not clear way in c++ to store and retrieve date/time.
any comment?
PostgreSQL TimeFormat 9+ version https://www.postgresql.org/docs/9.1/datatype-datetime.html
Which is a 8byte int(64 bit) time format in microsecond precision, UTC without timezone (from top of the table).
When you create a table you can either time-stamp the record by PostgreSQL current_timestamp , OR insert into table as integer 64bit microsecond format. since PostgreSQL has multiple time format you should decide time any the format you want from table
PostgreSQL approach CREATE,INSERT,RETRIEVE
"CREATE TABLE example_table(update_time_column TIMESTAMP DEFAULT CURRENT_TIMESTAMP)"
"INSERT INTO example_table(update_time_column) VALUES update_time_column=CURRENT_TIMESTAMP"
"SELECT (EXTRACT(epoch FROM update_time_column)*1000000) FROM example_table"
C++ approach
auto/int64_t cppTime = get64bitMicrosecondFormat from some library`
something similar to this answer: Getting an accurate execution time in C++ (micro seconds)
Then push your object / record to PostGRESQL, when retrieve in microseconds, adjust precision /1000 for milliseconds etc.
Just don't forget to synchronize PostgreSQL and C++ timestamp length (eg. 8byte - 8byte each side) otherwise, your time will be thresholded either side, and you will lose precision / get unexpected time.
Django attempts to address the timezone problem by storing dates internally in UTC and converting them to the client's timezone for display. This sounds fine and good in theory, until you realize two major things:
Many timezones can and do exist inside of the same UTC offset.
Since there are no timezone HTTP headers, we need to determine the timezone of the client manually, and this requires the use of JavaScript. However, JavaScript can only reliably determine the UTC offset of the client and may not guess the correct timezone.
With these two problems in mind, I assume a simple solution would be to ignore timezones, DST, etc. altogether and rely instead on the client's current UTC offset. On each page load, JavaScript on the client would update the client's cookie with the client's current UTC offset and middleware in Django would load that value for each request.
Here is the problem: Django makes use of get_current_timezone() which retrieves it's data from the value set when timezone.activate() was last called. timezone.activate() takes a timezone object as an argument.
Is there a way to use timezone.activate() with only a UTC offset?
The solution you describe, of getting the client's current UTC offset and sending back to the server, either via a cookie, or some other mechanism, is a common approach. Unfortunately it's flawed. Just because people do this doesn't make it a good idea.
The problem is that the offset you gather from the client is for a specific moment in time. However, you may not be working with that same moment in time on the server.
For example, you might call new Date().getTimezoneOffset() on the client, which gives you a value of 480, which is 480 minutes West of UTC, or UTC-08:00 (note the sign inversion). So you pass 480 to the server, load a date from the DB in UTC, and apply the offset. Except, perhaps the date you loaded was from several months ago, and the client's offset for that date was UTC-07:00. You have therefore applied the wrong offset, and produced a resulting value that is an hour off from what it should be.
A time zone cannot be identified by an offset alone. A time zone identifier looks like "America/Los_Angeles", not just UTC-8. This is a very common mistake. Read more under "time zone != offset" in the timezone tag wiki.
There are only two correct ways to handle this scenario:
Use a library like jsTimeZoneDetect or moment-timezone to guess the time zone of the browser, then let the user pick their time zone, defaulting to the guessed value. You can then use the selected or guessed time zone in your server-side code with Django or whatever.
Send only UTC to the client, do the conversion from UTC to local time in the browser using JavaScript. (The browser understands the behavior of the local time zone where it is running, even if it has trouble identifying it.) The catch here is - older browsers might possible convert older dates incorrectly, due to this bug. But for the most part, this is still a reasonable approach.
This strange behavior has recently came to my attention, while I was testing my Rails app on local environment in which I use around_filter to set the timezone to registered user (the default timezone is UTC).
What I did was that I registered a new user in my app. My current time was 10pm GMT-5 (March 3), and this user's created_at time was saved to database to 4am UTC (March 4). Now, I know that this time is saved in database with the timezone settings, but here comes the problem:
I use a graph for visual representation of daily registered users, and when I called the following function to tell me number of users registered in the last few days:
from ||= Date.today - 1.month
to ||= Date.today
where(created_at: from..to).group('DATE(created_at)').count
It would say that this user was registered in March 4, while it was in fact registered on March 3 from my perspective.
My question is:
How should I call where function and group by a created_at column, so that the dates with be affected correctly (according to my timezone) ?
Or is there something else that I should be doing differently?
I'm not a rubyist, so I'll let someone else give the specific code, but I can answer from a general algorithmic perspective.
If you're storing UTC in the database, then you need to query by UTC as well.
In determining the range of the query (the from and to), you'll need to know the start and stop times for "today" in your local time zone, and convert those each to UTC.
For example, I'm in the US Pacific time zone, and today is March 7th, 2015.
from: 2015-03-07T00:00:00-08:00 = 2015-03-07T08:00:00Z
to: 2015-03-08T00:00:00-08:00 = 2015-03-08T08:00:00Z
If you want to subtract a month like you showed in the example, do it before you convert to UTC. And watch out for daylight saving time. There's no guarantee the offsets will be the same.
Also, you'll want to use a half-open interval range that excludes the upper bound. I believe in Ruby that this is done with three dots (...) instead of two (at least according to this).
Grouping is usually a bit more difficult. I assume this is a query against a database, right? Well, if the db you're querying has time zone support, then you could use it convert the date to your time zone before grouping. Something like this (pseudocode):
groupby(DATE(CONVERT_TZ(created_at,'UTC','America/Los_Angeles')))
Since you didn't state what DB you're using, I can't be more specific. CONVERT_TZ is available on MySQL, and I believe Oracle and Postgres both have time zone support as well.
Date.today will default to your system's set timezone (which by the way should always be UTC, here's why) so if you want to use UTC, simply do Time.zone.now.to_date if rails is set to UTC
Otherwise you should do
Time.use_zone('UTC') do
Time.zone.now.to_date
end
After this you should display the created_at dates by doing object.created_at.in_time_zone('EST')
to show it in your current timezone
I'm reading timestamp fields from a PostgreSQL database. The timestamp column is defined as:
my_timestamp TIMESTAMP WITH TIME ZONE DEFAULT NOW()
When reading from the database, I convert it to a boost timestamp like this:
boost::posix_time::ptime pt( boost::posix_time::time_from_string( str ) );
The problem seems to be that boost::posix_time::time_from_string() ignores the timezone.
For example:
database text string == "2013-05-30 00:27:04.8299-07" // note -07 timezone
boost::posix_time::to_iso_extended_string(pt) == "2013-05-30T00:27:04.829900"
When I do arithmetic with the resulting ptime object, the time is off by exactly 7 hours. Is there something better I should be doing to not lose the timezone information?
I think you should be using boost::local_date_time, which handles time zones. There is an example in the documentation that is very similar to what you're trying to do: http://www.boost.org/doc/libs/1_41_0/doc/html/date_time/examples.html#date_time.examples.seconds_since_epoch
EDIT: Boost supports date parsing with specific formats. http://www.boost.org/doc/libs/1_40_0/doc/html/date_time/date_time_io.html#date_time.format_flags
string inp("2013-05-30 00:27:04.8299-07");
string format("%Y-%m-%d %H:%M:%S%F%Q");
date d;
d = parser.parse_date(inp,
format,
svp);
// d == 2013-05-30 00:27:04.8299-07
I originally asked this question so many years ago, I don't even remember doing it. But since then, all my database date/time code on the client side has been greatly simplified. The trick is to tell PostgreSQL the local time zone when the DB connection is first established, and let the server automatically add or remove the necessary hours/minutes when it sends back timestamps. This way, timestamps are always in local time.
You do that with a 1-time call similar to this one:
SET SESSION TIME ZONE 'Europe/Berlin';
You can also use one of the many timezone abbreviations. For example, these two lines are equivalent:
SET SESSION TIME ZONE 'Asia/Hong_Kong';
SET SESSION TIME ZONE 'HKT';
The full list of timezones can be obtained with this:
SELECT * FROM pg_timezone_names ORDER BY name;
Note: there are over 1000 timezone names to pick from!
I have more details on PostgreSQL and timezones available on this post: https://www.ccoderun.ca/programming/2017-09-14_PostgreSQL_timestamps/index.html