I believe that I have arrived at a very opportune moment.

我想我到得正是时候。

The offer could not have come at a more opportune moment.

那个建议提得正是时候。

He arrived at the most opportune moment.

他在最适当的时候到达。

A more opportune moment might never arise.

恰当的时机可能不会再有了。

This is an opportune moment to remind him.

这恰是提醒他的时候。