I'm having trouble with the time aspect in activity charts

[MedTechCD]

Specifically selections or are you happy with exact intervals?
Read here for some tips:

Intervals vs Selections & Activity Zoomed view

What is a selection? How/when to make a selection? What can it do for you? A selection in Intervals is a zoomed in view of a part of an activity. You can make a selection by click-hold-drag-release at a point in the chart. The important thing here is that you must be in the active part of the plotted area. Not above in the header part because it won’t work there. Once you do that you will get a zoomed-in view of the selected part. To zoom back out to the complete activity, left click anywhere i…