on the dot

The train leaves at 10.00 on the dot so we need to leave soon or we'll miss it.

'On the dot' means exactly at the time specified.

on the dot
時間通りに

Larry arrived at 6:00 on the dot. 
ラリーは6時きっかりに到着しました。

The meeting started at 3 o'clock on the dot. 
会議は3時きっかりに始まった。

It's five o'clock on the dot.
5時ちょうど［ぴったり・きっかり］です。