The Indian classical tradition assigns hours to its measures. Some rags belong to dawn, some to midday, some to the night, some to a season, and performing one outside its time is a departure that a trained musician notices immediately. The Guru Granth Sahib uses thirty-one such measures and does not arrange them by hour.
That creates a standing question about how the volume was meant to be used, and it has a practical edge. A continuous reading of the Granth from beginning to end, which is a recognised observance in Sikh practice, moves through the rags in the fixed order of the volume, not in the order of the clock. A congregation singing from the volume in sequence is therefore not observing the traditional hours, and a congregation observing the hours is not reading in sequence.
Both things happen. Practice accommodates the tension rather than resolving it, and different lineages of musicians have taken different views of how binding the traditional timing is on scriptural material.
What the volume itself requires is unambiguous and narrower: the hymn is to be sung in the measure named in its heading. Nothing on the page prescribes an hour. Whether the hours are part of what the Gurus intended, or a convention of the surrounding musical culture that the Granth does not adopt, is a question about performance rather than about text, and it belongs to the living tradition to answer rather than to a reference work.