# Our PHP TimeTraveler to address DateTime::modify() pitfalls

Everybody knows the PHP native method `DateTime::modify()`

The `DateTime` extension supports awesome modifiers like "2 days ago", "last day of this month", ... or basic ones like "+1 month".

But the last one is actually tricky: it increments the month value of the date.

For instance 2023-01-01 (January first) becomes 2023-(01+1)-01 => 2023-02-01

But as not all months last 31 days: incrementing one month on 2023-08-31 results to 2023-09-31 which doesn't exist and is thus corrected to 2023-10-01.

If you keep playing, you will notice that `modify('+1 year')` returns a different date than calling 12 times `modify('+1 month')` 🤯

```php
(new DateTime('2023-01-31'))->modify('+1 year'); // 2024-01-31
(new DateTime('2023-01-31'))
    ->modify('+1 month') // 2023-03-03 which is the root cause
    ...
    ->modify('+1 month'); // 2024-02-03
```

At AssoConnect, we deal with monthly and yearly subscriptions, memberships, ... so we need reliable and predictable date operations.

The latest release of our [https://github.com/assoconnect/php-date](https://github.com/assoconnect/php-date) library brings a month & year traveler with 3 main features:

1. The last day of the month is sticky
    
    September, 30th + 1 month = October, 31st *instead of 30th*
    
2. A month is never skipped
    
    January, 30th + 1 month = February, 28th (or 29th) *instead of March, 1st*
    
    February, 29th + 1 year = February, 28th *instead of March, 1st*
    
3. The year-over-year result is consistent with the initial reference
    
    January, 30th + (1 month) x 12 = January, 30th *instead of February, 2nd*
    

We published this library under the MIT license so feel free to use it!