See the following code:

function printDiff($tz) { $d1 = new DateTime("2015-06-01", new DateTimeZone($tz)); $d2 = new DateTime("2015-07-01", new DateTimeZone($tz)); $diff = $d1->diff($d2); print($diff->format("Year: %Y Month: %M Day: %D"). PHP_EOL); } printDiff("UTC"); printDiff("Australia/Melbourne");

The result is:

Year: 00 Month: 01 Day: 00 Year: 00 Month: 00 Day: 30

Questions:

- How can the difference between the same day of two adjacent months (1st of June and July) be different than 1 month?
- Why does the computation mode depend on which timezone I use if there is no DST between the given days, there is no leap year, there is nothing special?
- Where can I find the exact description of the algorithm how the difference is computed?

## Answer

The *date* extension stores the time values in GMT. The GMT offset is stored separately. It is applied at the late phases of calculations, for correction.

Particularly, `DateTime::diff`

internally calls `timelib_diff`

function, which calculates difference between two dates, applies the DST correction, then normalizes its internal structures, in that order.

When both dates are in *UTC*, the function compares the following:

`Timestamp=1433116800, year=2015, month=6, day=1, hour=0, minute=0, second=0`

`Timestamp=1435708800, year=2015, month=7, day=1, hour=0, minute=0, second=0`

It subtracts years, months, days, hours, minutes, and seconds correspondingly. The difference is exactly one month. Therefore, the internal structures are not modified after normalization.

When both dates are in *Australia/Melbourne*, the function compares the following:

`Timestamp=1433080800, year=2015, month=5, day=31, hour=14, minute=0, second=0`

`Timestamp=1435672800, year=2015, month=6, day=30, hour=14, minute=0, second=0`

Both dates are obtained by subtracting 10 hours (the timezone offset without DST correction). As we have seen, the DST correction is applied after subtraction of the time values, if needed (it is not needed, in particular). The difference before normalization is:

0 years, 1 month, -1 day, 0 hours, 0 minutes, 0 seconds

Since we have an offset between the months, normalization function computes the difference as

0 years, 0 months, 30 days, 0 hours, 0 minutes, 0 seconds

The `DateInterval::format`

method is straightforward. It just substitutes the placeholders to the members of `timelib_rel_time`

structure. That’s why we get *1 month* for UTC, and *30 days* for Australia/Melbourne.

**The a format character**

That looks somewhat unreliable. However, there is a `days`

member in the `timelib_rel_time`

structure, which always equals to the difference in days. The value is available via the `a`

format character:

function printDiff($tz) { $d1 = new DateTime("2015-06-01", new DateTimeZone($tz)); $d2 = new DateTime("2015-07-01", new DateTimeZone($tz)); $diff = $d1->diff($d2); print($diff->format("Year: %Y Month: %M Day: %D days: %a"). PHP_EOL); } printDiff("UTC"); printDiff("Australia/Melbourne");

*Output*

Year: 00 Month: 01 Day: 00 days: 30 Year: 00 Month: 00 Day: 30 days: 30

**P.S.**

The values in this answer are obtained with the help of `TIMELIB_DEBUG`

macro.