Capital gain tax on property sale

I booked residential property in 2015made some advance payment and got allotment letter on making advance payment . The position of flat will be given in march 2021. The balance payment of 40 lacs is to be made before I get possesion / the builder makes sale deed iny name
I sold my existing (old) property (which was purchased in 2007)in December 2020. Capital gain on this old property which was sold in dec.2020 works out to be 30 lakhs. please guide can I get the exemption / benefit. Also inform what will be the date of purchase of new property date of allotment which is dated 2015 Ordate of sale deed which will be march 2021
Thanks in anticipation