Should you say "0.9 second" rather than "0.9 seconds"?

Solution 1:

At the computer magazines where I worked for many years, we had a strict house rule to identify measurements of less than one with a singular unit of measure. For example, we would say

¾ inch or 0.75 inch

not

¾ inches or 0.75 inches

This style decision required endless enforcement because many of our writers habitually used plural units of measure with measurements of less than one.

One style guide that shares our old house style preference is Words into Type, third edition (1974), which offers this advice:

Decimals. For quantities of less than one, a zero should be set before the decimal point except for quantities that never exceed one. Also, when units are not abbreviated the singular is used for quantities of one or less. ... [Relevant examples:] 0.32 second, 0.75 grain

The Chicago Manual of Style, sixteenth edition (2010) evades this question almost completely. But to the extent that it has anything useful to say on the subject, it tends to contradict Words into Type. Here is the most direct remark it makes on the subject:

10.68 Plural form for abbreviations of US measure. Abbreviations of US units of measure, like their scientific counterpart, are identical in the singular and plural. ... Note that the unit of measure in such expressions as 0.5 yd. and 1.5 yd. is generally pronounced as if it were plural (i.e., point five yards; one point five yards).

This last statement seems to suggest that Chicago endorses writing out 0.5 yd. as 0.5 yards rather than as 0.5 yard—although it never explicitly says so. It's a very odd treatment, when you think about it: Chicago explains how 0.5 yd. "is generally pronounced" by US English speakers but never specifies whether US writers should spell out 0.5 yd. as 0.5 yard or as 0.5 yards. From this episode, you might think that the book was called The Chicago Manual of Pronunciation Style.

The Oxford Guide to Style (2002) sides with Words into Type on this matter:

7.2 Units ... Elsewhere [that is, aside from instances where units help form compound adjectives, as in "a two-mile walk"], units are pluralized as necessary, but not if the quantity or number is less than one: two kilos, three miles, 0.568 litre, half a pint.

If I recall correctly, part of the rationale for using singular units of measure for quantities less than one was that it helped avoid misreadings of, for example, ".8 pound[s]" as "8 pounds." (That is also a sound reason for including the zero before the decimal point, by the way.) But more than anything else, I think, we were swayed by the idea that a number consisting of a fraction less than one would certainly be attached to a singular unit (for example, ⅝ inch), so it wouldn't make sense to treat the exact decimal equivalent of ⅝ (0.625) as taking a plural unit of measure (inches) instead of a singular one (inch). But obviously, reasonable (and unreasonable) people differ on this question.

Solution 2:

I think the rule as written differs significantly from the common usage.

It is normal to use the plural "seconds" in all cases.

As @WS2 identified, the phrase "of a second" is also fairly common when refering to a fraction of a second less than 1.