Meanings

at the critical moment

opening hours (of a store, restaurant, etc.)

closing hours (of a store, restaurant, etc.)

usage time

(approx.) four o'clock (am or pm, old time system)

two o'clock (old time system)

(approx.) six o'clock (am or pm, old time system)

(time of) emergency

spare no effort while you are young

during pregnancy

cesium atomic clock

space-time

atomic clock

Greenwich Mean Time

Greenwich Time

Universal Time

ill-gotten gains fade as fast as they were obtained

a few hours

clock to be placed on tables, bookshelves, etc. (as opposed to a watch)

sometime