The word תמיד (always) means that the matter will be without interruption. And there is תמיד that is definite without stopping a second, and there is ordinal תמיד which occurs at a special point in time and does not change, like every morning or every Sabbath.

And this is the difference between תמיד (always) and כל הימים (all of the days): כל הימים means that the matter will not be interrupted during a specific time period, while it does not indicate that it is תמיד, not definite תמיד and not ordinal תמיד. And תמיד means that it is permanent, while it does not mean that it will not be interrupted during a specific time period.