Hyphenate adjectives, not nouns

"Object-oriented" is hyphenated, but "use case" is not.

run time — noun, the time at which the system runs (not hyphenated)
run-time — an adjective describing something which exists at run time (hyphenated)
runtime — noun, the run-time support, e.g. the VM (portmanteau word combining "run" and "time")

Wrong: ”Run-time errors are caught by the run-time at run-time.”

Right: ”Run-time errors are caught by the runtime at run time.”