The factors that influence the time of harvest are:
- The culture harvested - rice takes more time than rye.
- Climate seasons - according to water or to warmth.
- The light seasons. That factor together with the second one can cause funny results. For example, if we compare the spring in the Czech Republic and in the region around Moscow, according to the warmth the May in CR corresponds to June in Moscow. But according to the light, it is vice versa. So, the blooming of strawberries or of lindens in CR is a month earlier. But the the leaves on the birchs appear about at the same time or even later in CR.
- The time of seeding - if the climatic seasons are shifted, the same culture will be simply planted/seeded and harvested earlier in one place than in the other. Places far from sea has the temperature peak a bit after the middle of astronomic summer (21th June in the Northern hemisphere). Places closer to sea has that peak moved to later dates - it takes time to warm the sea.
Of course, the time of seeding could be defined voluntary, at will of the owner. In the places with very soft climate, as Canaries or Southern Chile, practically anything could be planted/seeded at any time with the same result. Here it is somebody's decision that determines the start of growth and thus the harvest time, too.
- How close the weather is to the ideal for the culture - the better the weather - the shorted is the time for riping. Too cold, too hot, too wet or too dry (without watering) weather can slow the growth.
The lighting difference for Cyrenaica and Greece is practically insignificant - they both are far from poles.
The points 2 and 5 depend on the slow longtime changes of the climate and thus are the subject of history. For nowadays Cyrenaica is the part of dezert or very close to it. And it was not in the antic times. So, the agriculture timings now are much more strict. And the time for riping is longer, than in Greece, if you do not water the culture. So, nowadays, it is not so sure about earlier harvesting in Cyrenaica. On the contrary, you could wait for your wheat forever.
As for the antic times, when the Northern Africa grew the wheat for the whole Rome Empire, the desert regions were far from the coast. The region was only somewhat dry.
The temperatures of 350BC, according to 1850-year cycles, should be similar to these of 1500AC, or were at the lower end. So, the winter should be noticeable. And Cyrenaica's inhabitants had to adapt the starts/ends of agriculture cycles to the same season changes, as in Greece. Of course, you are absolutely right, the warmer weather meant earlier spring seeding and harvesting. The dry summer moved the harvesting time in the same direction. They in Cyrenaica had to harvest their spring wheat much earlier.
It is not so simple with the second harvest. I am not sure Greece had one in that climate. So, it is unclear, what is to be compared.
And I don't know if Cyrenaica had enough summer rains to seed wheat in July and not to wait for the September rains.