Building Payroll System Without Gusto or ADP?

Question

For example, if our company wants to build payroll processing on our own without using any third party tool like ADP or Quickbooks or Gusto or Paychex by connecting directly to US IRS, and each of US state systems and do it ourselves.What APIs, providers and Open source systems - we can use.

jqpabc123 · Accepted Answer

There is very little in the way of Open Source that competently addresses or supports US payroll. What exists is typically limited "demo ware".
Which is probably just as well unless you plan to Open Source your system too.
Beyond the simple spreadsheet, there is a fairly broad ecosystem of specialized or support intensive business software that falls into a "no fun" category that gets ignored or dismissed by Open Source. This is what keeps MS Windows on the desktop and service providers in business.