The verb ser is used to express the time of day. Use es when referring to "one o'clock" and use son when referring to all other hours.
Es la una = It's one o'clock.
Son las dos = It's two o'clock.
The feminine article (la, las) is used before the hour because it refers to "la hora."
Es la una. = It's one o'clock.
Son las dos. = It's two o'clock.
Minutes can be added to the hour using the word y (and).
Es la una y cinco. = It's five minutes past one.
Son las tres y doce. = It's twelve minutes past three.
Minutes can be subtracted from the hour using the word menos (less).
Es la una menos cinco = It's five minutes till one.
Son las tres menos doce = It's twelve minutes till three.
You can also use the words media (half) and cuarto (quarter).
Es la una y media = It's half past one.
Son las dos y cuarto = It's quarter past two.
Son las tres menos cuarto. = It's quarter till three.
To say something occurs at a specific time, use the formula a + la(s) + time.
La fiesta empieza a las nueve. = The party begins at nine o'clock.
El banco abre a las ocho y media. = The bank opens at half past eight.
To differentiate between a.m. and p.m. use the expressions de la mañana, de la tarde and de la noche.
Son las dos de la tarde. = It's two in the afternoon.
Son las dos de la mañana. = It's two in the morning.
Son las diez de la noche. = It's ten in the evening.
When no specific time is mentioned, use the expressions por la mañana, por la tarde, por la noche.
Siempre leo el periódico por las mañanas. = I always read the newspaper in the morning.
Here are a number of useful time expressions:
por la mañana = in the morning (no specific time)
de la mañana = in the morning (specific time)
por la tarde = in the afternoon (no specific time)
de la tarde = in the afternoon (specific time)
por la noche = in the evening or night (no specific time)
de la noche = in the evening or night (specific time)
la mañana = morning
el mañana = tomorrow, future
mañana por la mañana = tomorrow morning
pasado mañana = the day after tomorrow
ayer = yesterday
anoche = last night
la noche anterior, anteanoche = the night before last
el lunes que viene = next Monday
la semana que viene = next week
el año que viene = next year
el lunes pasado = last Monday
la semana pasada = last week
el año pasado = last year
al mediodía = at noon
a la medianoche = at midnight
alrededor de = around
de día = days
durante el día = during the day
a tiempo = on time
en punto = exactly, on-the-dot
tarde = late
temprano = early
