leftout purchase bill

hi
I purchased some services and got bill in january manth, same taken for accounts and TDS purpose in feb month

but on making gstr3b january and feb, i leftout taking ITC of that bil

1. whether i can take the ITC in against may 3b return,
2.for which month i can consider for purchase return(gst 2 or 2a)
3. what is the consequences as I considered the bill for diff accounting year,(i.e fy 1718 &1819)