«أَنْتِ طَالِقٌ فِي غَدٍ»
“You are divorced within tomorrow.”
Fi does not necessarily identify every part of the named day as the exact instant of the effect. This opens the question whether a particular portion of the day—such as its end—was intended. In Abu Hanifa’s analysis that intention can be relevant.
Without fi, “you are divorced tomorrow” is more directly tied to the start of the next day. This is not a stylistic difference but one concerning the time when a divorce takes effect.
The particular wording, its context and any legally relevant intention have to be assessed together rather than imposing one rule on all temporal expressions.