1 I don't have suggestions to change formatting options on a numeric column beyond the predefined formats
(one could kludge a concatenated text solution with date time functions year, month, day, hour .... for cosmetic purposes only)
2 Round can work if you transform the units first.
JMP time units are seconds If you divide this by 60, the resulting number is in minutes, divide by 15 to change the units to 15's of minutes, then round, then change units back to seconds. Formula would look like Round(:Column/(60*15)) * (60*15)
Once the rounded time is known, a table summarize could potentially group similar time blocks together. This may meet your goals for aggregation of numeric values.
PS- instead of the "round" option, consider the "floor"- time groupings are often labelled based on the earliest time in the group, instead of the average.