Delta t (dt), PERiOD.

$$ Sam Wormley writes tomgee:
So it doesn't matter what the definition of constant
velocity is, to you, right? You will define it and use
any way you want to, right? After all, it's a free
country and you can make up anything you want, right?

There is only one definition dv/dt = 0

$$ Where, dv/t = x ..and, v/dt = y.
[ SPEED is ANY PATH/duration ; VELOCiTY the VECTOR/duration. ]

$$ You need BOTH velocity vector END-POiNT positions at ONCE.
$$ The VELOCiTY is ALWAYs known _ONLY_ in retrospect, so 'dv'
$$ caN'T EVER be known, at ONCE (i.e. ..at ANY *SiNGLE* 't').

$$ Atomic giga-frequency counters of REAL time.
The BEST atomic clock is SiMPLY a GiGA-frequency-COUNTER.!!
And, even the VERY best ATOMiC clocks CANNOT be SYNCRONiZED
TOGETHER, at once, on any GR or any OTHER sort of Ph.Tivity
TiME-line (i.e. WORLD-line), in ANY SPACE-time-continuum.!!

$$ All "clocks" are simply various FREQUENCY-COUNTERs used
even since very early on in PRE-history (to wit, period
tally-marks ..on excavated sticks & bones), to "TRY-to"
*CLOCK* ..the "REAL TiME".!! NOT to mention SUN & MOON.

However, a sort of TiME-AVERAGED simultaneity is achievable
..all having to compensate (REset) for design mysteries.!!

$$ DELTA v, (dv), caN'T *HAPPEN* ..all at ONCE.
$$ Simultaneity happens AT ONCE; EveryTHiNG else needs TiMiNG.
$$ And, *REAL* TiME is why *SHOP (LAB)* TiME has *ERROR* bars.

$$ ANY clock MOViNG ..or MOVED any LOWER ..will run FASTER.!!
$$ ANY clock MOViNG ..or MOVED any HiGHER, will run SLOWER.!!
$$ ANY clock's HORiZONTAL "SLOW transport", is UP transport.
$$ ANY clock moving OUT [ HORZ ] on a turn table", moves UP.

$$ ANY clock VARiES in ALTiTUDE for ANY horizontal TRANsPORT.

$$ Three VELOCiTY vector END-POiNT positions REQUiRED, for dv.
$$ Three VELOCiTY vector END-POiNTs (3 positions) define a dv.
$$ [GR Tivity CRACKED-pots: NOTE 'dt' caN'T HAPPEN ..at ONCE].

$$ Sam Wormly ..delta t (dt), PERiOD.
"It takes an infinite amount of time ..at which point,..".
-- Sam Wormley.