"A esa hora ya habré llegado"
"Debería haber terminado para octubre"
"Él dijo que estría aquí a esta hora"
This is a little bit hard to explain for me, but I'll try. "By" could most of the times be translated to spanish as "por", but when translating every sentence where it is used, "por" will not be an adequate preposition for using nor a good translation; sometimes there is not even a preposition involved. Let me explain:
In the case of "You can achieve anything by studying hard", it would be translated as "Puedes alcanzar cualquier cosa estudiando duro". Any preposition was used there.
Let another case be "You need to get here by car", it would be translated as "Necesitas venir en carro". The preposition "en" was used instead of "por".
The obvious case is when the translation is "por". So, you don't always have the same translation of "by", sometimes you don't even use it. So, in order to translate it correctly, you need to think about which preposition could be used. Most of the times in spanish, and even in english, when talking about time it is understood (it seems, I've never thought about this before) that you are inside a frame of time. So you are "en una hora", "en octubre", "en un rato". However, when you say "now" you can use "ahora", or in this case "a esta hora", because you can be precise about the time. "a" is a very versatile preposition.
You can use "a" when you state some definite time "a esta hora", " a las 10:00". Also, you can use it when stating a distance in amounts of time "está a una hora de aquí". However, when talking about a frame of time, you use "en" as stating that something will happen in that interval of time, or past that time: "en una hora llegaremos", "en la mañana".
Yet another disclaimer: if you say "en la mañana siguiente" (being in that frame of time) you can also say "a la mañana siguiente" (as moving the time forward, "a" as "hacia").
I always recomend http://dle.rae.es/ as a dictionary.
Hope this can help you :).