"It is seven fifty in the morning."
Translation:Son las siete y cincuenta de la mañana.
It's "de" because of the use of prepositions in Spanish. "De" is used for many purposes and among them there's the use for time. I'll put some examples in Spanish about how to use "de": pertenencia, materia , procedencia y origen, cualidad, contenido , modo, profesión, causa, tiempo, condición o participación: El auto de Pedro es nuevo. En casa de herrero, cuchillo de palo. De una sentada, se bebió todo el vaso de vino . Entrando al área de penal, se tiró de espaldas. Juega de centro delantero. Está que se muere de amor. Se duerme mejor de noche. De no haber comido tanto, te sentirías mejor.
It is only in our head that the use of many prepositions "belong" where we think they do. It is much more a tradition than a "fact". For instance many people in American English say "on" accident rather than the more accepted "by" accident even though we also favor "on" purpose and few people say "by" purpose. The more you try to figure out the "why" the farther down the rabbit hole you'll fall. It is not math and it's only one step up on spelling.
I'm not positive about this, but just this morning, I was wondering why numbers are feminine gendered when talking about time, masculine otherwise usually. What I'm guessing is that the "las" refers to hours (fem.), which are not stated but rather implied. The reason for the "son" is because it refers to more than one hour of time. It is one o'clock = "Es una [hora]." I hope this helps, and if it is not correct, someone please chime in.
If you want to say, "it's ten of eight in the morning" (as opposed to calling it seven fifty), you could say, "son las ocho menos diez de la mañana." (Not sure if it's on the list of accepted translations for this sentence -- some of the language trees will let you do it either way, and some won't -- but it's correct Spanish.)