When Assessing Officer is​ liable to determine the Advance tax liability?
If a taxpayer who has a legal obligation to pay advance tax fails to make payment for advance tax or advance tax is lower than the correct amount AO may pass an order asking the taxpayer to pay tax on assessee's current year income
Such order shall clearly specify the amount payable and in number of instalments the same needs to be paid
Such order should be passed anytime during the previous year but before 1st March, i.e., by 28th February
Computation by the Assessing Officer

Assessing Officer can serve an order requiring the assesse to pay advance tax if he is of the opinion that such person is liable to pay advance tax or the tax paid is lower 
In such cases, the Officer will take the higher of following incomes and calculate the tax as per prevailing rate of income-tax:-​
(a)The total income of the latest previous year in respect of which Officer has assessed income, i.e., the year for which an assessment has been completed by the Income-tax Officer,

or

(b)   The total income declared by the assesse in any return after the year of assessment by officer., i.e., Any income furnished by the assessee in Income-tax return for any previous year after