The phrase "be as honest as the day is long" is an idiom that exudes the importance of being truthful and sincere. There are numerous synonyms that one can use to convey the same message. "Speak the truth" is a simple and direct alternative to this idiom. "Be sincere" is another option that emphasizes the genuine nature of honesty. "Be forthright" suggests a straightforward and direct approach to communication. "Be transparent" emphasizes the importance of revealing all aspects of a situation or oneself. Finally, "be genuine" emphasizes the necessity of being true to oneself and others. Using any of these synonyms can help to deliver the message of honesty and sincerity.