I’m digging through the account_asset
module and got everything working like expected. But then I walked into a problem. I bought in the middle of last year some equipment and want to add them as asset
. I asked my accountant what they calculated as deprecation amount, so I can check if Tryton does the same. Unfortunately it doesn’t.
To illustrate the difference in calculation I take the following variables:

value
= €1200,– 
start date
= 1st of September (01092019) 
end date
= 31th of December (31122023) (we deprecate over ~5 years) 
residual value
= €0,–
What (the software of) my accountant calculates:
Even the first year is not complete their software will take the whole year in account so:
deprecation amount per month
= 1200 / 5 / 12 = €20, / month
Because the in the first year we start in September it will take 4 months to deprecate the asset. So the deprecation amount 1st year
will be €20 * 4 = €80,–
The difference (€160,–) will be spread over the 4 coming years.
What Tryton does is a bit different. Tryton calculates how many days are left and then uses that to calculate the deprecation amount:
days_left
= 122 (from 1st of September till 31th of December)
first_year_ratio
= 122 / 365 = 0.3342
last_year_ratio
= 365 / 365 = 1 (because it’s a whole year)
Tryton calculates the deprecation ratio per year
as
5 years  2 + first_year_ratio + last_year_ratio
= 4.3342
deprecation amount 1st year
= (€1200 / deprecation ratio per year
) x 0.3342 = €92.54
As you can see around €12.5 difference.
I’m also a bit baffled by the calculation Tryton makes. Is this something described anywhere? Am I missing something?
Anyhow, I want to comply with the software of my accountant which means that I have to make some changes. Normally I would do that in a module, but maybe others have the same issue so it can maybe end up in the standard module.