var islemList = (from isl in entities.Islemler where (isl.KayitTarihi.Date >= dbas && isl.KayitTarihi.Value.Date <= dbit) selec
if KayitTarihi is a date column in DB (and dbas and dbit are DateTime), use:
KayitTarihi
var islemList = (from isl in entities.Islemler where (isl.KayitTarihi >= dbas && isl.KayitTarihi <= dbit) select isl);