I agree with Larry, and to further emphasize: It's not the one-columness that is important, it is the single correct time-value. There's very little point to pushing non ISO8601 dates and times together. The advantage only surfaces if you can deduce from that a proper date-time value such as ISO8601 or even a Julianday or GMT seconds or such, so that you have one field that gives a specific point in time with which you can calculate precedence, duration, order, containment and the like and format it easily with the existing time-format functions into whatever output you or your users may fancy.