Answer:
24 days.
Explanation:
Since the clock loses 15 minutes per day, it will show the correct time after gaining back 24 hours, equivalent to 1 day.
Therefore, the clock will show the correct time for the first time after 24 days, which is calculated as follows:
One day to gain back the first 15 minutes
One day to gain back the second 15 minutes
...
One day to gain back the 24th 15 minutes
So the answer is 24 days.