It means: Is the one.
"La maestra es una pendeja" translates to "The teacher is an idiot" in English.
It's 1:14 pm.
"Cuando es la una y media" means "when it is one thirty" in English. It is asking for the specific time of 1:30.
It's one o'clock
"Es la una en" means "It is one o'clock in" in Spanish. It is used to specify the time in relation to a particular location or time zone.
Yo la conozco; es una amiga. I know her (I recognize her); she's a friend
"It is a celebration in the last Thursday of November."
Es la Una, that is not the way to say one o'clock in Spanish it is la Una en punto
Cuando is not acutually part of es la una y media. es la una y media is the time. Cuando means when and es la una y media is 1:30 PM in Spanish.
"Es la una en punto" means "It's one o'clock exactly" in Spanish.
It is one in the afternoon.
if you're talking about time on the clock, una menos nueve means 1 minus nine, or figuratively 12:51