Sorry, available market cap data for Berkshire Hathaway (BRK.A) starts on January 4, 1988. Berkshire Hathaway (BRK.A) has a market capitalization of $1T, based on 1.44M shares at a price of $724,605 (down 1.3% today).

DATE	OPEN	HIGH	LOW	CLOSE	VOLUME
December 31 1985	$2,470.00	$2,480.00	$2,470.00	$2,470.00	200
December 30 1985	$2,480.00	$2,490.00	$2,480.00	$2,480.00	200
December 27 1985	$2,500.00	$2,510.00	$2,500.00	$2,500.00	90
December 26 1985	$2,510.00	$2,510.00	$2,510.00	$2,510.00	30
December 24 1985	$2,510.00	$2,530.00	$2,510.00	$2,510.00	200
December 23 1985	$2,525.00	$2,530.00	$2,525.00	$2,525.00	300
December 20 1985	$2,480.00	$2,480.00	$2,480.00	$2,480.00	800
December 19 1985	$2,600.00	$2,625.00	$2,600.00	$2,600.00	90
December 18 1985	$2,625.00	$2,625.00	$2,625.00	$2,625.00	300
December 17 1985	$2,625.00	$2,625.00	$2,625.00	$2,625.00	90
December 16 1985	$2,585.00	$2,585.00	$2,580.00	$2,585.00	200
December 13 1985	$2,620.00	$2,620.00	$2,620.00	$2,620.00	200
December 12 1985	$2,600.00	$2,615.00	$2,580.00	$2,600.00	200
December 11 1985	$2,610.00	$2,630.00	$2,580.00	$2,610.00	400
December 10 1985	$2,630.00	$2,630.00	$2,600.00	$2,630.00	30
December 09 1985	$2,600.00	$2,630.00	$2,600.00	$2,600.00	70
December 06 1985	$2,625.00	$2,630.00	$2,625.00	$2,625.00	300
December 05 1985	$2,600.00	$2,630.00	$2,600.00	$2,600.00	900
December 04 1985	$2,630.00	$2,630.00	$2,600.00	$2,630.00	100
December 03 1985	$2,630.00	$2,630.00	$2,630.00	$2,630.00	10
December 02 1985	$2,600.00	$2,630.00	$2,600.00	$2,600.00	50
November 29 1985	$2,625.00	$2,630.00	$2,600.00	$2,625.00	200
November 27 1985	$2,630.00	$2,630.00	$2,600.00	$2,630.00	200
November 26 1985	$2,630.00	$2,630.00	$2,600.00	$2,630.00	80
November 25 1985	$2,600.00	$2,630.00	$2,600.00	$2,600.00	100