kanozyo ha nebousita hi ha, matiawase no zikan wo kaete moraukoto ga arimasu.

They are marking two different topic layers.

• 彼女は = As for her...
• 寝坊した日は = As for days when she oversleeps...

This is very common in Japanese. The sentence first sets the overall topic (her), then narrows it down to a specific situation (days when she oversleeps).

So the structure is roughly:

• As for her,
• on days when she oversleeps,
• there are times when she has the meeting time changed for her.

The second は often gives a sense of setting the condition/frame for what follows.

Because 寝坊した日 is a relative clause: a clause directly modifying the noun 日.

So:

• 寝坊した日 = the day when she overslept / a day on which she oversleeps

In Japanese, verbs before nouns often appear in plain form and act like adjectives. The past form here does not mean the whole sentence is in the past. It just describes the kind of day:

• 寝坊する日 would sound more like a day on which she oversleeps in a general/future-type sense
• 寝坊した日 refers to a day where the oversleeping event has occurred

In natural English, this often gets translated more smoothly as on days when she oversleeps.

Because は is being used to make that time phrase the topic or contrastive frame.

Compare:

• 寝坊した日に = on the day when she overslept
  This is a simple time marker.
• 寝坊した日は = as for days when she oversleeps / on days when she oversleeps
  This sets up a condition or situation for the rest of the sentence.

So 日は is not just marking time. It is more like:

• When it is a day she has overslept...
• On days like that...

This use of は is very common after time expressions.

待ち合わせ means an arranged meeting, especially meeting up with someone at a set time/place.

So:

• 待ち合わせの時間 = the time of the meetup / the arranged meeting time

It is not just any abstract time. It specifically refers to the time that has been set for meeting someone.

変えてもらう means to have someone change it for you or to get someone to change it.

Breakdown:

• 変える = to change something
• 変えてもらう = to receive the favor of someone changing it

So the nuance is:

• 変える = she changes the meeting time herself
• 変えてもらう = someone else changes the meeting time, and she benefits from that / requests it

In context, it implies she asks the other person to adjust the meeting time.

The sentence does not explicitly say, because Japanese often leaves that understood from context.

With 〜てもらう, the viewpoint is from the person who receives the favor. So here:

• 彼女 is the one who benefits
• someone else changes the time for her

Most likely, that someone is the person she is meeting or the other party involved in the meetup.

So the hidden idea is something like:

• She gets the other person to change the meeting time for her.

Because こと turns the action into an event/occurrence, and then あります says that such an occurrence exists.

So:

• 変えてもらう = to have someone change it for her
• 変えてもらうこと = the event of having someone change it for her
• 変えてもらうことがあります = there are times when that happens / sometimes that happens

This pattern is extremely common:

• verb + ことがある = there are times when... / sometimes...

It is a standard way to talk about occasional actions or experiences.

Because ことがある is a set pattern built on the idea of existence.

• ある = to exist, to occur
• こと = an event, a matter, an occurrence

So ことがある literally means something like:

• there exists such an event

That is why が is used: it marks what exists.

So:

• 変えてもらうことがあります = There are occasions when she has the time changed for her

No. It usually means sometimes, there are occasions when, or it happens from time to time.

So it suggests:

• not always
• not never
• just occasionally

If the speaker wanted to say it happens regularly or usually, they might use something stronger, such as:

• よく
• いつも
• たいてい

But ことがあります keeps it at the level of occasional occurrence.

It can often function more naturally in English as on days when she oversleeps.

Japanese frequently uses a noun like 日 to mean the occasion/day on which something happens, and in context it can refer to repeated situations, not just one specific calendar day.

So here, even though 日 is singular in form, the sentence as a whole is understood habitually because of ことがあります.

That is why the natural interpretation is something like:

• On days when she oversleeps, she sometimes has the meeting time changed.

Yes, and that would also be natural:

• 彼女は寝坊したときは、待ち合わせの時間を変えてもらうことがあります。

The difference is small, but the nuance shifts a bit:

• 日 focuses on the day/occasion
• とき focuses more directly on the time/when

In many contexts, both work.
Here, 日 gives a slightly more concrete feel: on days when this happens.

The sentence as a whole is best understood as present habitual.

Why?

• 寝坊した is past form, but it only modifies 日
• あります is present
• ことがあります indicates something that sometimes happens

So the overall meaning is not she did this once in the past. It means:

• This is something that happens sometimes

That is a very common source of confusion for English speakers: a past-form verb inside a relative clause does not automatically make the whole sentence past tense.

A helpful breakdown is:

• 彼女は
  As for her,
• 寝坊した日は、
  on days when she oversleeps,
• 待ち合わせの時間を
  the meeting time
• 変えてもらうことが
  the event of having someone change for her
• あります。
  exists / happens

So the grammar skeleton is:

• [Topic] は
• [time/situation] は
• [object] を
• [verb-て + もらう] ことがある

This pattern is useful well beyond this sentence.