The correct spellings are --punctual --punctuality They both have the letter "C" in them. "Punctuality" is the same as "punctual" with -ity added. "Punctual" is an adjective. "He is always punctual." "Please be punctual." The correct spelling is "diligent." "He is a diligent person and punctual about arriving at work." We form the adverb by adding -ly: "punctually." You could say "I will arrive punctually at 4 o'clock." "I will arrive at 4 o'clock on the dot" is more natural. "Punctuality" is rare. It is a noun. it is the quality of being punctual. "Punctuality is important in personal relationships." "Three of her best qualities are cheerfulness, reliability, and punctuality." Sometimes we form this kind of noun by adding "-ness", sometimes by adding "-ity." The "punct-" root means "point" or "dot" and is found in a few other words such as --"punctuation," . , : ; ? ! --"puncture," I have a flat tire because it was punctured.