Hi Jason Sheldon,
Thanks for the post. We have received a couple reports of this in Esri Support and have recently logged a usability issue for it. ArcGIS Online will always store dates in UTC 0 and calculates the offset when the time zone box is checked so that the dates display in local time (your browser converts the date from UTC to local for display). Of course when overwriting/appending that time zone box isn't there, which is what we're looking at.
I think the best option is to convert the time from local to UTC in the spreadsheet beforehand. For example since you are Central Time (UTC -5 in Daylight Savings), if you have 5/22/2020 6:00 p.m. in the spreadsheet convert it to 5/22/2020 11:00 p.m. before upload. There are multiple options for this workflow in Excel.
If you have ArcGIS Pro, you can calculate the field using an Arcade Expression:
var startDate = ($feature.date_field);
var newDate = dateAdd(StartDate, 5, 'hours');
return newDate
Hope this helps - please send me a direct message or email pklingman@esri.com with your customer number and I can help get a support case going or attach your customer number to the issue.
-Peter