e-book What you need to know about dates and times in computing

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When you call the getDistance method on the distance record, it returns If you call format on the distance record, while your locale is somewhere in the UK, it prints out 2m. If you call format while your locale is somewhere in the US, it prints out 6 ft, 6.

How do computers keep time?

What do you say when someone comes to you and says that they wanted the distance object to represent distance in meters instead of millimeters? Now, when they call getDistance , it returns 2 which is what they wanted. You would rightfully be flabbergasted. Or maybe add a newInstanceMeters Integer meters function to your Distance class that multiplies the passed meters by and returns a Distance object representing 2 meters as millimeters.

With DateTime objects, the logic is the same. GMT is the model , and dates in all other time zones are representations. Apex actually makes this pretty easy for us — all DateTime objects we create are GMT DateTimes, but provide us a way to display it in any time zone. Just like our Distance class above which stores distance in millimeters, but can display it in any many different units.

Working with dates in JavaScript can be complicated. Learn all the quirks and how to use them.

What happens if I construct a date for February 10, PM? The DateTime instance we constructed represents a moment in time. What do we see in the console when we print out the date using System. Even though we constructed a DateTime for PM. What is the reason for this? Internally, Apex uses GMT as a model to store the date and time. And we can represent this moment in time in multiple ways, just like we could represent our distance as meters or feet and inches.

This is the result in the debug log:. The Salesforce world is filled with people asking how they can construct a DateTime in their local time zone. Most people who ask this question end up adjusting the DateTime object to look like their local time. The following is the result of this code:. Do you see the problem? This is the same mistake is doing Distance. So what is the right answer to the question of constructing a DateTime in your local time zone?

Make sure that your DateTimes represent the correct moment in time, and you can always display it in any time zone. It is a good rule of thumb to never use DateTime if you only need to represent a particular time or a particular date. Which gets saved as Feb 10, GMT. Which is all good.

He sees February 9, What happens next? Your boss, who graduated from Ohio U. Always be sure to use the Date and Time types in Apex and the Date and Time fields on sObjects if you only need to store a date or a time. Another case where you would want to use separate Date and Time objects or fields is when representing an event that takes place at a certain local time in more than one place. For example, if you want to store a deadline to submit documentation on February 1, at PM no matter where you are your team in New York submits at PM Eastern Time and you team in Los Angeles submits at PM Pacific Time , you would want to use separate date and time fields.

Remember, DateTime represents a specific moment in time that is automatically adjusted across different time zones.

Dates and Times in R

In , the US passed a bill which would extend daylight saving time by one hour effective A good solution to deal with situations like these would be to persist future DateTimes representing a particular local time in the local time zone , but unfortunately Salesforce only allows us to save DateTimes in GMT.

We could talk about saving a local DateTime as a Date, Time and a String Timezone field, but the added complication and the lack of ability in Apex to directly construct a DateTime from data in any given timezone I intend to write a blog on how to do this later , it might not be worth it. Depending on the situation, what we can do is keep an eye out for changes in time zone rules and be ready to either inform users or update the saved dates in case the rules do change. Almost everything discussed above becomes obvious if we follow just one principle: DateTime always has GMT as the model.

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The definitive guide to JavaScript Dates

Toevoegen aan winkelwagen Kopen Toevoegen aan verlanglijstje Van verlanglijstje verwijderen. Its sole purpose is to quickly get you up to speed on this ubiquitous and commonly misunderstood topic. This is the first in a series of Cromulent Guides.