Time runs more slowly in a moving frame — a stationary observer measures a longer interval than the proper time.
Symbols
Δt
measured time
[s]
γ
Lorentz factor
Δt0
proper time
[s]
Where it comes from
A moving clock runs more slowly from the point of view of a stationary observer. The effect becomes noticeable only at speeds close to c (muons, GPS satellites).
When to use it
Comparing time in frames moving relative to each other.
Example
The proper time is Δt0=1 s and γ=1.25. Calculate the measured time.