What's the difference: 'sometime' vs 'some time'

ScottsEnglish

I was recently reading a student email.  In it, she wrote the following:

We surely need to plan another holiday some time next month.

What's wrong here?  Well, the problem is in her use of the words 'some time'.

Rule 1: ‘Sometime’ means ‘at a time in the future or the past that is not known or not stated’. It can answer the question 'when?' 

E.g., He said he'd sit his TOEIC test sometime = a time in the future which is not stated.

Rule 2: 'Some time' means 'quite a while'. It can answer the question 'how long?'

E.g,, I think I'll spend some time preparing for my English test tonight = quite a while (i.e., several hours,) studying.

Which rule has she broken in her email?  Write your answer in the comments section below.