Which is the best option of the two

[vvvvvvizard]

If Qlik Sense autodetects your dates leave it as is , implicit conversation 

Or like the old days when Date( Date#.....

To explicitly state it 

Which is best practice coding 

Don't bother explaining date function I kn0w how it works 

Back up your answer thou 

[BrunPierre · Accepted solution]

My preference would be to lean towards explicit definition. In my experience date formats can be ambiguous and to avoid misinterpretation, I always resort to explicitly defining formats. And perhaps, the most important and what I believe to conform to best practice is ensuring that date time fields are consistently interpreted in the same way regardless of changes in environment variables or data sources.