I sold stock that resulted in a short-term gain. 10% of these sales proceeds were held back by escrow and paid out 12mo later. Are the holdback proceeds ST or LT gain?

This pertained to an investment in a private company that was sold (merged with) a public company. The merger and sale agreement contemplated 10% of the sales proceeds as "holdback" to be distributed one year later after satisfaction of certain conditions included in the purchase and sale agreement. The original gain was short-term in nature (April purchase and September sale in same year) but the holdback was distributed 12 months later.