Adding center to the column directive resolves this.
#(define ((time-parenthesized-time up down upp downp) grob) (grob-interpret-markup grob (markup #:override '(baseline-skip . 0) #:number(#:line (
(#:center-column (up down)) #:vcenter "(" (#:center-column (upp downp)) #:vcenter ")" ))))) -- Dave Higgins -- Any philosophy that can be put "in a nutshell" belongs there. -- Sydney J. Harris
<<attachment: dave_higgins.vcf>>