If you earn 10,000 diamonds every day, and you currently have 53,000 diamonds, then you already have 53,000 + 10,000 = 63,000 diamonds after one day.
This means that in one day, you can earn 10,000 diamonds and have a total of 63,000 diamonds.
To reach your goal of 400,000 diamonds, you need 400,000 - 63,000 = 337,000 diamonds.
Therefore, it will take you a total of 337,000 / 10,000 = 33.7 days to reach your goal.
Since you have a time limit of 51 days, you will have 51 - 33.7 = 17.3 days left over.