Yes Jon, SUM(C:G) to refer fields range C to G on a row, is NOT supported yet. Will make note of this pattern and consider it for our backlog.
Regarding the alternative, SUM(C,D,E,F,G). Yes, unlike ranges it will not expand to include any new columns inserted between columns C & G. However, we ensure that it still works on the same set of columns i.e. lets say you are inserting a newly column at F, now formula will evaluate as SUM(C,D,E,G,H).
On re-launching Date-type Edit dialog, you will notice that formula expression also now reads: SUM(C,D,E,G,H). Newly inserted column does not affect this column formula definition and it still evaluates the same.