Since we only uses date fields in accounting, it does not have any sense to have a datetime for currency rate:
- it is error prone for the users as the timezone they're in will affect the rate (and possibly take yesterday's rate).
- it involved the possibility to create several rates per day, which was not fully supported and might give undeterministic results (so we also add a constraint in order to only have one rate per day)
qdp-odoo
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