Investors eyeing President Donald Trump’s Opportunity Zones face a ticking clock if they want to fully capture one of the biggest tax breaks in decades.

But few so far seem ready to make the leap.

Tucked into the 2017 tax overhaul, Opportunity Zones let investors reduce and postpone taxes on profits from stocks, businesses and investment partnerships provided the money is reinvested in one of more than 8,700 low-income communities across America. Investors can also avoid tax on future profits...