Calculation of deprecation amount with incomplete first year

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 (01-09-2019)
  • end date = 31th of December (31-12-2023) (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.

I do not think you can expect two softwares to compute exactly the same way but you can edit the depreciation lines.
But in your case, the accountant software is not computing the depreciation up to 31-12-2023 but up to 31-08-2023 to get exactly 5 years. And this is almost what you will get with those dates in Tryton with some differences for the initial and last amount (which will balance).

I can only create new lines by a click of a button. Those lines are readonly and the many2one itself is also readonly.

Hmm, that’s a nice suggestion. I asked my accountant about that. Waiting for an answer.

When I set the end_date in Tryton to 31-08-2024 (mention a year later because 2019 and 2024 should be treated as 1 year) and create the lines, I’m just a few cents off :smile: And that’s more then OK with me.

Indeed that’s true. It is because we can not ensure proper values if we let user update them.

So please leave it as is.

Just got the answer of my accountant. It is indeed how their software is doing deprecation.

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