For hundreds of years, mottoes, cast or engraved unto the dial surface, have been a central part of garden sundial designs. Sundial mottoes are incredibly diverse in subject and tone. Sometimes they're written in the first person (ie "I am silent without the sun"), others express the passing of time ("Time flies") or the need to embrace the beauty of life, nature and love ("Grow old along with me, the best is yet to be").
Here are some of my favorite quotation about time although they've never been used on a sundial (to the best of my knowledge).
“Lost time is never found again” Benjamin Franklin
“Time is the coin of your life. It is the only coin you have, and only you can determine how it will be spent. Be careful lest you let other people spend it for you.” Carl Sandburg
“Time expands, then contracts, all in tune with the stirrings of the heart.” Haruki Murakami
“Now, five years is nothing in a man's life except when he is very young and very old." Pearl S Buck (The Good Earth)
“Time is a great teacher, but unfortunately it kills all its pupils" Berlioz
“Enjoy life. There's plenty of time to be dead.” Hans Christian Andersen
“Nature does not hurry, yet everything is accomplished.” Lao Tzu