on the button
1 (informal, especially North American English) at exactly the right time or at the exact time mentioned
We arrived at 4 o'clock on the button.
2 exactly right
You're on the button there!
e.g.
Early. We're leaving the house at 8 a.m. on the button. 明日の朝 8時出発だからな (/yupeco/『ホーム・アローン』)