Update from the JP:
Relaxation of 'expatriate' income tax
(...)Under the prevailing 2008 Income Tax Law, every individual (including foreign nationals) who has resided in Indonesia for more than 183 days within a 12-month period, or has resided in Indonesia for a full tax year and intends to remain in Indonesia, is considered a domestic taxpayer. Otherwise, the individual is considered a foreign taxpayer and is not obliged to pay income tax in Indonesia.
The wording "183 days within a 12-month period" irritates me a bit...since the exact wording is "a 12-month period" and not "a calendar year", this means that Indonesia does not consider a whole calendar year, so that a person can be a tax resident only for a part of a calendar year as well?
For example, a foreigner and the Indonesian spouse arrive in Indonesia in September 2019. That would mean:
(a) In 2019, they are not tax residents in Indonesia and, therefore, in the beginning of 2020 they do not have to declare or pay taxes for 2019?
(b) But after 183 days of not leaving Indonesia (in this example, somewhere in March 2020) they become tax residents. This means that, in the beginning of 2021, they would have to declare and pay taxes on their 2020´s income from March 2020 - December 2020 and not the income of the whole year 2020 (therefore, January and February excluded since in those two months they were not tax residents yet)?