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Using DateTime with precision

The only class in the PHP SPL to work with dates and times is DateTime (and it's immutable counterpart). It represents a specific moment in time in a specific timezone (whether that timezone/offset is explicitly defined or not). Unfortunately dates are complex and there is more than just a moment in time. For example there are

• time of day which is relevant on every day (like business hours).
• a specific date (like Christmas) which isn't bound to a timezone.
• a subset of a date time like time, date, month or year.

Basically, every time you're not talking about a specific moment, the DateTime classes contain not just more informationen then needed, but even misleading information.

This bundle / package tries to solve this issue by introducing a wrapper value object Moment for a moment in time and value objects for the more precise sub sets like Time, Date, Month and Year. It's only a thin wrapper over DateTime and uses it internally for all modifications and comparisons. This way you don't have to make sure that your DateTime is

• at a specific date to compare times.
• at midnight to compare dates.
• at the first of the month to compare months.
• at the first of the year to compare years.

Additionally, the package provides a streamlined way to have the system running in UTC but still do the modifications in the relevant timezone. The internal DateTime is always in UTC and only internally converted to the relevant timezone for modifications. This way you can be sure that you're not missing or receiving an hour due to a switch of summer-time to winter-time in the relevant timezone.

This Symfony bundle includes Symfony normalizers for automatic normalization and denormalization and Doctrine types to store the objects directly in the database.

As it's a central part of an application, it's tested thoroughly (including mutation testing). Currently, more than 80% of the lines of code in this repository are tests.

Installation and configuration

Install package through composer:

⚠️ This bundle can be used (and is being used) in production, but hasn't reached version 1.0 yet. Therefore, there will be breaking changes between minor versions. I'd recommend that you require the bundle only with the current minor version like composer require digital-craftsman/date-time-precision:0.10.*. Breaking changes are described in the releases and upgrade guide.

When would I need that?

Basically whenever you use a DateTime object for something other than a single moment.

Storing more information in those cases just lead to more questions, like "When storing the month, do we store the first day of month at midnight, and if so, in which time zone?" and therefore increases complexity. Additionally, you need mutate or reduce the point in time to be able to compare it. With the package it will be as easy as:

$now is a Moment (in UTC) and $facility->openFrom is a Time (in the timezone of the facility).

The idea is that your system can run in UTC and all moments are in the timezone UTC. But all values that have an implicit time zone like a date or a time of day will be stored with just the data needed. This way we're getting rid of additional data that creates more surface for possible bugs. Through precise value objects and specific comparison functions, the code is more readable than before.

$now is a Moment (in UTC) and $facility->earliestDayOfBooking is a Date (in the timezone of the facility). The same method isBeforeInTimeZone that is used previously for the time comparison is the same that is used here. Depending on the type of the second parameter, the comparison is done on the relevant part of the moment.

Modifications work the same way.

The resulting $bookingsAllowedFrom is still a date time with timezone UTC but the modification is done in the relevant timezone.

Integration

For the best code readability, it's best to use the Moment provided with the package as a full replacement for \DateTime or \DateTimeImmutable when you're speaking about a moment in time and the others value objects for the rest. The package provides normalizers and Doctrine types for Moment and all parts.

The Doctrine types are automatically registered with the bundle with the following type names:

• dtp_moment
• dtp_time
• dtp_date
• dtp_month
• dtp_year

The package also contains a clock component consisting of the interface Clock with the two implementations SystemClock (for general use) and FrozenClock (for testing). The SystemClock will be autowired for the Clock and automatically replaced with FrozenClock in the test environment.

Design

Immutability

All mutations on the Moment and its parts are immutable.

Contribution

The local setup is build with Docker and controlled through Make commands. Run make to see all available commands and what they do.

Before creating a PR or pushing any code, please run make verify to run all tests and validations locally.