Many G-4 visa holders who apply for green cards incorrectly assume that their US tax residency will begin on the day they receive their green cards. In some cases, this is true. But in most cases, your residency will begin before that. That’s because once you retire, your days in the US start to count toward the US Substantial Presence Test of residency. And your residency start date is the earlier of the day you receive your green card or the day you were first physically present in the US during the year.
So, if you were a G-4 visa holder living in the US who retired in November and received your green card in March, your first date of residency would be January 1. This is the first day after your retirement that you were physically present in the US in the year you received your green card. This is a general rule, and there are many factors to consider. Just be careful! Some retirees have run into unintended immigration consequences by traveling outside the US to delay their residency start dates.