HP 54710A User's Reference Manual page 219

Measurements
Time-interval measurements
The faster the transition, the smaller the time error introduced by dc level
errors. This is true both for edges of the same polarity and for edges of
opposite polarity, as well as for channel-to-channel measurements, as
illustrated by figure 13-15. If the edges have a non uniform slew rate with
fast and slow regions, try to reference the time-interval measurement to the
fastest-changing region as shown in figure 13-16.
Figure 13–15
Faster Transitions Decrease Effect of dc Offset Errors
13–34