Here's some CVI code to build hh:mm:ss from excel time.
// xlTime is the double extracted from an excel cell containing time.
// time String will contain hh:mm:ss
double xlTime;
char timeString[20];
double xlHours, xlMinutes, xlSeconds;
xlHours = floor(xlTime * 24);
xlMinutes = floor((xlTime * 24 - xlHours) * 60);
xlSeconds = floor((((xlTime * 24 - xlHours) * 60) - xlMinutes) * 60);
Fmt (timeString, "%s<%d[w2p0]:%d[w2p0]:%d[w2p0]",
(int) xlHours, (int) xlMinutes, (int) xlSeconds);