if (SysUtils.DateToStr(DateTimePicker1.Date)<>'1900-1-1') and (SysUtils.DateToStr(DateTimePicker3.Date)<>'1900-1-1') then
filterstr := filterstr+ ' and (TXRQ between '''+SysUtils.DateToStr(DateTimePicker1.Date)+''' and '''+SysUtils.DateToStr(DateTimePicker3.Date)+''')';