Thank you for reaching out to us.
This is an excellent question.
When a woman performs two
bedikot on each of the
clean days, which is preferred practice, then the boundaries of morning and afternoon are as you wrote. Performing
bedikot in the morning and afternoon creates a sort of frame for counting the day, showing that it begins and ends without bleeding.
Even so, if a woman were to perform her first
bedikah late or her second one early, it would not interfere with her clean days.
Additionally, it is acceptable, though not generally preferred, for a woman to perform one
bedikah per day. In that case, a
bedikah performed between
chatzot (halachic noon) and
minchah ketanah would still count fully as a
bedikah for a given day.
Please let us know if we can be of further assistance.
