ST-12 appears to be a Wisconsin sales and use tax return.
Due on April 15th are your federal and state income tax returns. The federal return is form 1040 for an individual and the state return is apparently named form 1.
Two totally different taxes, procedures, agencies, filing deadlines, etc.
This is something you’ll need to get used to if you’re in business. I did the math once and the typical Seattle employer has 14 different forms to file with 7 different tax agencies. Other states are not all that different.
When you make sales, you are collecting sales tax from the customer as an agent of the state. The ST-12 form is for you to send in those taxes you have collected.
On April 15, you will file your income tax, which is tax on your own income, or in the case of your business, the profit from the business. It has nothing to do with sales tax collected.
INCOME TAX RETURN DEADLINE FOR TAX YEAR 2011 - It is Tuesday, April 17, 2012
The traditional tax return filing deadline is April 15 of each year, but April 15, 2012 is a Sunday and April 16, 2012 falls on Emancipation Day in the District of Columbia.