Retrieve the value for tax calculation as that of the currency

Actually it is only available to have a Tax that is not dependent on the time, Why is It is not correct?:
i.e.,
-- You are on March 20,
-- your Tax is 15%,
-- you make and invoice and then,
-- the law changes and the tax is 20%,
-- you have to keep doing invoices,
"-- you have two options --",
one, change the name of the tax, and the percentual value of the tax, or add more tax with the right names and the values,
but, if you add a way to keep the track of the value changes of the tax per dates basis as that of the currency, you could, easily just add a line for the day and the value, be it, fixed, percentual, balance or python code. and you keep with only one tax that is applied based on the time.

So this way if you know law changes from a day on, you just has to add a single line and the one who is going to invoice, if not aware of the changes, it is not prone to errors. Another example, is when you have to many products depending on that tax is changing, if there are many products, it could be a onerous job to done, with this solution the work is reduced again to just change/add a single line, and from that day on, it is applied.

when you have 10 or 20 customers, is imposible attend all of them in the ame date only to apply an update on taxes.

Somebody could complain about the refund invoice. this has two point of view, even more, but as far as I know you could be winning or losing money because of the change of the tax and the tax rate to be considered to apply to the refund invoice, so one way to address the issue is relating the refund invoice to the date of the invoice you are refunding, so that way you could be refunding your invoice at the same amount rate of that day. but this a way of addressing this issue there could be other more elegant way of doing it.

There are a branch linked with the proof of concept, the branch only has related the account module, replace you current link to account module an link this to try it.

The changes on this branche aren't doing anything else than show the new models necesaries for the change. you can use a diff command on linux to see _only_this changes.

I will attach a "bzr diff" result done on my main branch.

So this way if you know law changes from a day on, you just has to add a single line and the one who is going to invoice, if not aware of the changes, it is not prone to errors. Another example, is when you have to many products depending on that tax is changing, if there are many products, it could be a onerous job to done, with this solution the work is reduced again to just change/add a single line, and from that day on, it is applied.

Somebody could complain about the refund invoice. this has two point of view, even more, but as far as I know you could be winning or losing money because of the change of the tax and the tax rate to be considered to apply to the refund invoice, so one way to address the issue is relating the refund invoice to the date of the invoice you are refunding, so that way you could be refunding your invoice at the same amount rate of that day. but this a way of addressing this issue there could be other more elegant way of doing it.

Conclusion:

OpenERP must be prepared for the future not only in technical ways even in functional ways, in _all_ countries around the world the laws around the taxes can change the value for a tax, your ERP system must be prepared to manage this changes around the dates.