You would be better to use one that was accurate. In the example shown the 90th day and therefore the one when a report would be due is the 28th of June. Earliest date would be 14th so they got that right even with the incorrect day 90, so that calculation is wrong too. Final day is Saturday 5th so the calculation is correct but as Immigration is closed at the weekend it should show Monday the 7th.
Like a lot on the internet, garbage.
1 day in March
30 in April
31 in May
28 in June
Total = 90.