John Sakamoto, citing webexhibits.org, writes,
Daylight Saving Time is indeed grammatically correct, as is Daylight Savings Time. The analysis is, however, faulty as the caveat implies. If anything at all is being saved, it's daylight, not time. So, to say that 'saving' is modifying 'time' strikes me as very odd.
I see two possible analyses. One is that it is time for saving daylight, analogous to time for reading the paper. Under this strategy you get daylight-saving time, analogous to paper-reading time. The s doesn't work here.
The other strategy is to analyse saving(s) as a noun, as in a one-time saving or a savings account. Under this strategy, daylight is a noun modifying the second noun saving(s). Now, if we look at the analogous cost saving(s), we find that the plural is more than twice as common as singular, but neither is rare.
But we're not finished. The noun phrase, daylight saving(s) is itself modifying time. While there are cases where a plural noun is used as a modifier (e.g., antiques show), it is far more common for such modifiers to be singular. Indeed, with our analogous cost saving(s), we find cost saving measures is roughly five times as common as cost savingS measures. Still, this doesn't rule out the possibility of the plural form if one wants to emphasise that there is a saving every day, as discussed here.
By the way, the daylight is saved for many individuals who would otherwise be sleeping through it in the morning, thus wasting it.