creating custom form error handling

im tracking events which have a few groups of related data, like
dates, times and tickets.
the issue im running into is when multiple forms make up one piece of
validation:
ex: if this is checked, these fields need to be present and one needs
to be greater than the other...
if one of these things is not right, id like to change two label type
fields where these input fields are grouped.

im guessing i cannot use the built in validation tools to handle these
errors, since it only seems to want to highlight the label and field
in question.
are there any good examples of how to group input elements and handle
errors so that one error highlights all relevant fields?

for example, if the end time is before the start time, i want to
highlight my time form elements along with error message

You'll need to write a custom validation, I think, if you want to
highlight more than 1 field for a given condition.

if end_time < start_time
  errors.add :start_time, 'your message here'
  errors.add :start_time, 'your message here'
end

What I have never tried is to hightlight a field without a message so,
in your case, having a unique error message (and no extra empty lines
in the messages section) but 2 fields highlighted at the same time.