The Malay language has two words of fascinatingly-ambiguous temporal description: 'kelmarin', and 'lusa'.
Depending on which state in Malaysia you hail from, 'kelmarin' can mean one or all of three things:
The day before yesterday
Several days before
In colloquial Malay usage, some folks (particularly older folks) tend to use 'kelmarin' as a synonym for yesterday.
But prescriptively, it is valid usage for 'kelmarin' to refer to both 'yesterday' and also 'the day before yesterday'. See Dewan Bahasa dan Pustaka:
(1) dua hari sebelum hari ini
/ "Hari ini hari Khamis, jadi ~ hari Selasa."
(2) hari sebelum hari ini
(3) beberapa hari yg lalu
Yet 'kelmarin' can vary in its standard meaning between different states (e.g. northern vs eastern vs urban), and even by the time of the day!
For instance, at night, Malays are more likely to use 'kelmarin' to refer to a time two days before, whereas saying 'kelmarin' at daytime can tend to mean 'yesterday'.
The opposite of 'kelmarin' is 'lusa' - i.e. the day after tomorrow ('esok').
'Lusa' can mean 'tomorrow'. But unlike 'kelmarin', 'lusa' more strongly implies a time two days after.
Dictionaries also recognize the ambiguity of the word 'lusa', to the extent of meaning an indefinite future time - i.e. sine die:
hari sesudah hari esok
/ "Hari ini hari Isnin, ~ hari Rabu."
a. besok ataupun lusa.
b. pd suatu masa kelak; lambat laun.
The English language has words of similar distinction - 'ereyesterday' ('kelmarin'), and 'overmorrow' ('lusa').
Fancy. Yet these words are archaic, and no longer seen, whilst their Malay counterparts remain in heavy use.
I remember as a child being confused about this difference between 'kelmarin' and 'semalam'. I had asked my mother, but received an equally confusing answer. So now, having done a bit more research, I've since learned:
Prescriptivist linguistic norms hardly ever apply to commonly-used words in Malaysia. 'Correct' Malay usage tends to defer to actual linguistic use.
Malays are quite comfortable with ambiguously-described time of events.
Since Malay words can differ from state to state in everyday use, when any colleague or person uses the word 'kelmarin'/'lusa' with you, best to ask for clarification immediately.