Business Tax Deadlines Partnership Returns:  due April 15th, can be extended until September 15th There are penalties for filing a partnership return late.  Filing an extension removes the late filing penalty (provided you file by September 15th).  The penalty for filing a partnership return late is $195 per partner per month.  Since a partnerhsip is a pass-through entity, the taxes are paid at the individual level.  So, any penalties for paying late will be charged to the individual partner. Corporate Returns:  due March 15th, can be extended until September 15th (for both C Corps and S corps) For S corps as well as C corps, there are penalties for filing late and for paying late.  The penalty for filing a late S corp return is $195 per shareholder per month.  Since an S corp is a pass-through entity, the taxes are paid at the individual level.  So, any penalties for paying late will be charged to the individual shareholder.  The penalty for filing a late C corp return is based on the balance due.  It's 5% per month, for a maximum of 25%.  The penalty for paying C corp taxes late is also based on the balance due.  It's .5% (one half percent) per month, for a maximum of 25%. Corporate Estimated Tax Payments (for calendar year  C corps):  due Apr 15, Jun 15, Sep 15, Dec 15 There is yet another penalty for underpaying estimated taxes.  Generally the C corp needs to pay 100% of the prior year tax or 100% of the current year tax, whichever is less.  There's no penalty if the balance due is under $500. Maryland Personal Property Tax Returns:  Due April 15th, can be extended until June 15th Corporations and LLCs must pay a $300 filing fee.  (Partnerships and sole proprietorships have no filing fee.)