CalFUSE converts wavelengths to a heliocentric reference frame. Does it convert the exposure start and stop times (EXPSTART and EXPEND) to a heliocentric frame as well?
No. CALFUSE does not convert the exposure start and stop times to a heliocentric frame of reference. The header keywords EXPSTART and EXPEND are genuine geocentric modified Julian dates, i.e., Julian dates with 2400000.5 subtracted.