We distinguish two basic temporal modalities, namely sometimes and always.
The sometimes temporal modality states that the relevant fact is true in at least one of the corresponding chronons at the finer granularity, or in at least one of the chronons of the interval in case an interval is given. For instance: ``The light was on yesterday afternoon,'' meaning that it was on at least for one minute in the afternoon (assuming minutes as chronons).
The always temporal modality states that the relevant fact is true in each corresponding chronon at the finer granularity. This is the case, for instance, of the sentence: ``The shop remained open on a Sunday in April 1990 all the day long'' with respect to the chronon granularity of hour.
This issue is relate to attributes varying within their validity intervals.