Tax depreciation is calculated monthly for most tax methods, the exception being 'annual' tax methods, which vary from jurisdiction to jurisdiction.
For example, in the USA, MACRS depreciation methods are calculated on an annual basis so we don't apportion them monthly.
If you need to calculate monthly figures - the easiest way to work this out would be to run a report to .csv (e.g. Asset Export, Asset Summary, Asset Group Summary or Depreciation Forecast) for the full financial year and then divide those figures by 12.
"But we prepare financial statements monthly and have a need for monthly tax depreciation"
AA generally doesn't offer the tax basis reporting for accounting/book purposes.
Having said that, in the USA for example, it is possible to closely mimic MACRS accounting methods.
If an asset is meant to depreciate over 5 years for tax, you can choose an accounting method (such as straight line) that depreciates over that same 5 years.
While there might be some variance between your tax and accounting values from year to year, they will balance out over the effective life of the asset.
If this workaround isn't acceptable and you need to strictly adhere to Tax Basis, the only way to get monthly figures out of AssetAccountant for accounting purposes would be to run tax reports across a financial year, export those to CSV/excel (using a report such as Asset Export or Asset Group Summary) and then apportion these across the applicable months, taking acquisitions and disposals into account.