Datum Inc. reported that its earnings for 1988 rose to $1.1 million, from $1 million a year ago. Sales reached a record $31.9 million, a 15% increase from the $27.8 million posted in 1987. The company’s 1988 sales and earnings include 6 months of sales from Austron Inc., which Datum bought last year. For the fourth quarter, net income totaled $186,000, which included $103,000 resulting from an accounting change. Net income for the comparable period a year ago was $241,000. Sales rose 22%, to $8.8 million from $7.2 million. Datum, based in Anaheim, designs and manufactures precision frequency products and timing instrumentation for military, industrial, space and commercial applications.