HP 54710A User's Reference Manual page 202

User Defined ∆ Time
On the oscilloscope, you may select the threshold level (lower/middle/upper),
polarity of edge (either rise or fall), and the edge number. The measurement
is then calculated as the time from the first source's edge to the second
source's edge.
This gives flexibility in defining ∆ time measurements between channels or
on a single channel between edges. To avoid confusing the delay
measurement with time base delay and trigger delay, the delay measurement
has been called ∆ time as shown in figure 13-11.
See also
Chapter 7, "Define Measure Menu," for information on delay measurements.
Figure 13–11
You may define ∆ time
Measurements
Timing Definitions
13–17