I must have misread/assumed the OP was working from home in South Dakota and was keeping his residency there.
Didn't know he would be going to and earning any income in Illinois.
I just looked it up, Illinois has a 30 day exemption for nonresidents earning income within the state.
After that all compensation earned within the state of Illinois is subject to state income tax on a per "working day" accounting.
Working day = any day that work done within the state of Illinois is greater than work done outside the state.
Travel time is not included UNLESS if traveling to Illinois for work was the only service for employer the nonresident was doing that day. (only the day of arrival is counted as a work day)
*This will likely be the biggest factor in your days worked in Illinois aggregating quicker than anticipated because you live so far from Illinois that anytime you have to go to the Office/Illinois for anything will at minimum count as 1 work day.*
This law (
SB1515 ) appears to have been designed to target all the people that live in Indiana/Wisconsin but work in Chicagoland. Maybe also for the Quad Cities with people living in Cockeyes but working in Illinois. Your employer will already have an accounting system/log for in state "work days",
BUT be sure to keep your own record of days worked in Illinois @hothouse_corn
If ever you feel its a toss up if the day should be counted = just count it. Always assume the State or the IRS will take the position that something is taxable or not exempt. The Illinois state income tax rate is only 4.95%, so my advice is that the risk/pain of getting audited is worse than whatever money you would have "saved" by not counting that 1 day. Don't let the tail wag the dog.
PS: sorry for the delay, while I was typing the "what counts as a working day" my 2 year old son threw up milk and cherrioes and what appears dried cranberries all over the carpet. Kitchen floor was like 1 foot away dude, get some accuracy one time for me. (look at this glamourous lifestyle I live, posting about state tax codes on a football message board and cleaning puke out of carpet on a Saturday morning)