HP 54710A User's Reference Manual page 217

Measurements
Time-interval measurements
Time-interval errors due to voltage errors cancel when you measure the time
between two edges having the same polarity, amplitude, offset, and slew rate
at the same dc level on the same channel. Note that this rule applies only to
single-channel measurements. For channel-to-channel measurements, the dc
errors on each channel could be different, so the errors may not cancel.
If the slew rates of the edges are different as in figure 13, there may still be a
time error introduced by the dc level error, though both times are referenced
to the same dc level and both edges have the same amplitude and polarity.
13–32