A woman may not immerse until she completes her seven clean days. Therefore, even if she or her husband will be out of town on the scheduled mikvah night, she may not immerse one night earlier.
If the couple cannot rearrange their schedule to enable them to be together on the scheduled mikvah night, the wife is permitted to delay her immersion until the night (or night before) she or her husband returns from the trip.
Please feel free to get back to us with any further questions.