Some clients are interested in setting a viewability threshold where duration would be lesser than one second (e.g. 0.5; 0.8 seconds). Also, some might find 1.5 seconds relevant. Currently both in 2.0 and NXT only integers can be selected (screenshot of selecting 0.5 as value: https://www.screencast.com/t/ht3b4wruuQAz)
Please note that if you try putting in 1.5 as a value, and click save, NXT will save it, without showing an error, but it will save 1 as a value. So it selects closest lower integer, although that is not what ones has put in.
Hopefully we can get this updated somehow :)