Last date of itr 31 july 2018, so what is four installment of advance tax means?

Please Elaborate. 

You are paying Advance tax for current. Year 18-19 . 

If the Income tax liability of any tax payer  in more than Rs 10000/- in a Financial year , then he is  liable to pay advance tax in installments during the year itself rather than paying this tax at end of the year .

Ok, Now I have a tax liability of more than 10000 in FY 2017-18, I am supposed to pay taxes in four installments so why 31st July is the last date?

If your tax liability  for the year  2018-19  more than 10000/- , you are suppose to pay advance tax  in  4  installments   and last  date is  15th march  2019 . You are paying advance ta for current  year  2018-19  not for 2017-18 , as you mentioned above . 

Ok thanks, but we have to assume total tax liability of current year my first installment is on 15th June and I have to pay 15% of estimated Tax liability, which I have to assume on 15 June till 31 March.

Let's say my estimated assumption is 10000/-, On 15 June I paid 15% i.e. 1500/- as the first installment. 

Now on year completion, my tax amount goes to 13500/-, Now I have paid less amount in installments. So I have to pay penalty u/s 234B &C,

Why?