Does Calm Emotions prevent you from becoming frightened/charmed for the whole duration after it was cast?

Calm Emotions reads :

[…] If a creature fails its saving throw, choose one of the following two effects.

You can suppress any effect causing a target to be charmed or frightened. When this spell ends, any suppressed effect resumes, provided that its duration has not expired in the meantime. […]

Initially, my logic is that a spell’s effect takes place when it is cast. Hence, the saving throw is forced upon the spell’s casting. You only apply the effect when the creature fails it’s saving throw.

This leads me to believe that only the effects causing a target to be charmed or frightened at the moment the Calm Emotions is cast are suppressed for the duration the spell. Otherwise, I’d expect the spell to be worded something like "For the duration, any effect that would cause an affected target to be charmed or frightened are suppressed".

However, I’ve seen a few posts and opinions saying that effects initiated after Calm Emotion is cast are suppressed as well…

RAW, which interpretation is correct? Which parts of/ precedent in the rules validate it?