Possible Duplicate:
0.25 mile or 0.25 miles?
When writing out a quantity and then a noun what is the correct form for decimals?
1 day
2 days
.5 days?
It seems that if you were to say "half day" then the singular is reasonable but if you were to day "point five days" that also seems correct.
You are right, one would say 'half a day'. One would also say '.5 of a day', but '.5 days'. The difference is that in the '.5 days', the verb day is described by a number other than one, in the other example, you've effectively got 'one .5 of a day', which means that day is qualified it with one, thus making it is singular.
