In English I can say :
2 d 3 h 2 m 20 s (= 2 days 3 hours 2 minutes 20 seconds)
What would be the abbreviations in Italian for day, hour, minute, second?
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I can say it could depend on the context you are talking about.
In everyday language, as pointed out in @RiccardoDeContardi comment, you can use giorno/i for day/days with an abbreviation like g./gg. and h (ore) , m or min. for minutes, and finally s or sec. for seconds.
In this blog post about La punteggiatura nel tempo (The use of punctuation in time) it is explained how it is used, for instance, in physics:
In fisica si misurano gli intervalli di tempo in giorni, ore, minuti secondi. Si usano le seguenti abbreviazioni: d (dies) per giorno, h (hora) per ora, m per minuti e s per secondi.
In physics we measure time intervals in days, hours, and seconds. We use the following abbreviations: d (dies, the latin form), h (hora) for hours, m for minutes and s for seconds.
Ovviamente si tratta di una notazione non molto adatta alla vita quotidiana: "Per favore quanto durerà il volo?" "2 h 10 m Signore". Molto più semplice rispondere "2 ore e 10".
Obviously this isn't a very suitable notation for everyday life: "How long will the fly be?" "2 h and 10 m, Sir". It's easier to reply "2 hours and 10"
Note that for the for the International System of Units the basic unit for time is the second (s) and other units of time may be defined as a multiple of the second. Obviously minutes ad hours are accepted and used due to their widespread use among non academic people.