Google Translate says
Nun wird ein Tag
but I think it does not feel correct. Can we just say "Heute..."?
Are there any other ways to say this?
Now a days smartphones are very common.
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Heutzutage is directly equivalent.
Heutzutage verwendet man Smartphones.
Heute verwendet man Smartphones.
This is not 100% the same when used without a context like früher.
There is another closely linked possibility:
Zurzeit verwendet man Smartphones.
This would change the meaning though, since it implies that the situation might end and I doubt anybody can imply that.
Sorry to interrupt, but the English expression is not now a days, but nowadays, which directly translates to "heutzutage" or "heute" or "in der heutigen Zeit". Keep going.
As obviously this is requested in the comments, I'm going to pick up your example, which should be corrected to:
Nowadays smartphones are very common
And could be translated as
Heutzutage sind Smartphones sehr gebräuchlich
Heute sind Smartphones sehr gebräuchlich
In der heutigen Zeit sind Smartphones sehr gebräuchlich
"Now a days" is not idiomatic English. Therefore, your example is not translatable, strictly speaking.
(If you really mean "nowadays", the answer would be "heutzutage" or any of the synonyms mentioned:
Heutzutage sind Smatphones / schlaue Mobiltelefone sehr üblich.
But that was not your question.)