Comorbidity refers to the existence of two or more diseases or conditions in the same individual at the same time.
While by definition comorbid disorders must exist at the same time, it may be the case that one of the conditions or disorders may have started before the other. For example, someone may develop PTSD in childhood, then later develop BPD as an adolescent. Or, both conditions may develop at the same time, but one may end (remit) while the other continues.
For as long as the symptoms of both disorder overlap in time, they are considered to be comorbid.