The periods here are search windows. They overlap, and they do not define simultaneous linguistic breaks everywhere. The date a text was composed, the date of the surviving manuscript, of the exemplar it was copied from, of a printed edition, of a recording and of a linguistic feature are different fields. A fifteenth century copy can preserve older material. A learned Byzantine form may imitate antiquity rather than record everyday speech.
The medieval and Byzantine programme
The Cambridge Grammar of Medieval and Early Modern Greek covers roughly 1100 to 1700. That window reflects the quantity and nature of accessible vernacular texts, and its introduction explicitly separates the language of the preserved texts from the inferred grammar of speakers B04 General Introduction pp.xvii–xix. That is why a "Byzantine" label does not on its own specify register, spoken grammar or a political boundary.
The section is organised in four dossiers: Late Antiquity and the early medieval period with their gaps; learned, ecclesiastical and administrative writing practices; later vernacular literary and documentary traditions; and regional differentiation into early modern descriptions. The epitome of the Kriaras dictionary is accessible through the Centre for the Greek Language B11 landing page. Athens Digital Glossa Chronos is a relevant cross period tool, but its stated release limits mean it cannot be assumed to be a complete, balanced corpus B12 Release status (v0.4).
Koine
The Centre for the Greek Language overview describes Koine as an interdialectal convergence B06 opening history paragraphs. That does not support a uniform speech map for the whole empire. Research studies levelling together with competing registers, such as literary Atticism.