According to the documentation for the time_field, you can define the format in the object's value, like "%T.%L" in order to allow input of milliseconds.
in my view I have:
.field
= f.label :start
= f.time_field :start, value: "%T.%L"
I also tried it this way:
f.time_field :start, value: "%H:%M:%S.%L"
but that too rendered the same way.
Documentation is sparse though: https://apidock.com/rails/v4.0.2/ActionView/Helpers/FormHelper/time_field

