Steps to reproduce:

1. Create a new host e.g. "A"
2. Create the first item with "Trend storage period" 366d
3. Create the second item for host "A" with "History storage period" 360d and "Trend storage period" 360d
4. Open host "A" item page and check the History and Trends filters

Result: 360 days are converted like 1 year.

Looks like logic is that 1 month = 30 days, so 12 x 30 = 360d or 1y.
Note: this problem appears in the range 360-364days.

Expected: 365d = 1y. 360d should be converted correctly e.g. 360d = 11m 26d.