Time is not a constant, as, time does not exist. Time is only relative to one's perception of it.
Time is not a Physical property.
Time cannot be slowed, sped up, reversed, bent, folded, stapled or mutilated, Even though I have done that on occasion,
Time is only the Duration of One Moment to the next as observed by a Sentient Being in a relative environment,
Time only exists, in a true form, when an intelligent being applies a relative measurement to the duration of a given moment.
The formula for time is,
Time = [Re + Rp] + [D x Rm]
[ Relative environment, + Relative perspective ] + [ Duration of a given moment multiplied by a Relative measurement ] = Time.
Time is only your Relative Perception of one moment to the next in your Relative Environment and has no bearing on any Physical Event, other than, the duration of the event moment.
If it appears too overlap, then that is only a 'glitch' in one's perception.
Added: to the TD; prove me wrong.
it overlaps every day at 4:20 .
Huh? The last time anyone thought time was contant it was still the 17th century.
Most of the time (i.e., in the past up to about 1900 A.D.), philosophy has agreed with physics: time is absolute: the Newtonian level of ascertaining and describing reality.
Since the advent of quantum mechanics (c.1900), time for physics has been proven a relativistic property.
Thus, your question ("if time is constant") could be modified by quantum physics.
Related: "The Physics of the Soul;"
"The Path of the Higher Self;"
"Light Is a Living Spirit;"
"A Fortunate Universe."
Time is relative. If I am on a plane and you're on the ground, we will experience time differently. It is definitely not constant, and most likely can be manipulated.
Time is not constant but relevant!
Einstein explained it perfectly!