Oh no!
Anyway…
No, meaning that it’s off by much less than a half-second. If you’re comparing time.gov to some other clock, and they differ by a half-second, it’s probably the other clock that’s off.
If you want the most accurate time, get it from GPS.
But, really - unless you are doing astronomical observations, anything better than 1 second is probably pointless.
And if you’re doing astronomical observations and the most precise timekeeping device you have available is your cellphone, then you probably should be working on grant proposals instead.
Not much less. I compared my desktop, laptop, tablet, and phone, and they were all clearly off from each other. Not quite half a second, but probably a third. The tablet was closest at maybe 1/10, but it was still easy to see them ticking over independently.