是的，some day 通常只指将来，即只用于将来时态。如：

Some day I’ll tell you. 有一天我会告诉你的。

Don’t bother to look for my umbrella; it will turn up some day. 别费心找我的伞了；它迟早会自己出来的。

They say they’ll stop dieting some day, but not right now. 他们说将来某一天他们将停止节食，但不是现在。

Perhaps some day it will be pleasant to look back on these things. 可能将来有一天我们回顾这些事时会感到愉快的。

有时虽然不是将来时态，但句子意思表示的是将来意义。如：

I hope we meet again some day. 希望我们将来有一天再见面。

I want to get married some day. But before that, I want to travel. 总有一天我要结婚。但在那之前，我要旅游。

另外，在特定语境中some day有时可指过去将来或虚拟语气。如：

I never realized that some day I would be living in China. 我从未想到将来有一天会在中国居住。

If he had lived, some day Michael would have done something important. 假定麦克尔当时在世的话，他本可做出了不起的事。