Cell's data format in numbers

I'm a part-time investor, investing in Japanese stocks in long terms 3~5 years. When I managed my portfolio in Numbers, I met a few problems and kindly ask your help in Apple Support Communities.

For example, I buy stock code 1997 on 2018/03/26 and sold it on 2021/05/11. So I input a simple formula F4-D4, which get period value at H4. However, data format of Duration only have wk or day, what I need is shown in years. in this case, shown as "3.1y".

So can anyone advise on how I can edit duration in years, but not wk or day?

thank you!