Open OnCall. Right-click on an assignment and select Shift Times.
The shift-times toolbox has a solid down-pointing black arrow that sets new default
hours for the service. First customize the times on the day the times change. Then
click the down-pointing black arrow to make the new times the default from that day
A small gray triangle in the upper-left of a cell denotes default hours that differ
from the times a shift is currently set to span. The gray triangles display for a
few weeks before the times change. If you move your schedule ahead and change the
default hours a 2nd time, the day the hours change from one old time to the next will
have a white tab in the upper left.