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Hello,

I encountered an issue with the following code:
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```php
echo Carbon::parse('2018-06-01')->subDay()->month;
```

Carbon version: **PUT HERE YOUR CARBON VERSION (exact version, not the range)**
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PHP version: **PUT HERE YOUR PHP VERSION**

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I expected to get:

```
6
```
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    to be used in comparisons, someone else may not, etc.
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But I actually get:

```
5
```
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    result you get.
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Thanks!