NEW YORK >> Apple, already the world’s most valuable company, hit the $600 billion level for the first time Tuesday.

Only one other company has been worth $600 billion —Apple’s old sparring partner Microsoft Corp. It reached that valuation for 13 trading days around the turn of the millennium, at the peak of the technology stock mania.

At its highest level, on Dec. 30, 1999, Microsoft’s valuation was $619 billion. It’s now worth $260 billion.

Apple shares hit $644 in morning trading, up 1.2 percent from Monday’s close.

Apple’s stock is up 59 percent since the start of the year, as investors have started catching up with what analysts have been saying: despite its enormous market capitalization, Apple’s stock has been undervalued in relation to its even bigger profits