A time interval is the distance or difference
between two timepoints on a timescale. If repeated regularly, intervals
can be measured in metric units, which are in the context of
seconds, minutes, hours, days, weeks, months, years.
An interval scale is a metric scale that
allows to measure quantity and difference of values in metric units.
In regard to time and date, in the context of
these values are timepoints on a timescale in units like seconds or days.
The term 'onset' is borrowed from music theory where it is used to mark the beginning of a note, but doesn't denote pitch or length. Likewise the onset in regard to timepoint generation for scheduling marks the starting of some action, without specifying the action itself or its duration.