It is reasonable to ask, but treat the original start date as your commitment until the firm approves a change.

Consulting firms often coordinate start classes, training, staffing, and immigration around specific dates, so flexibility can vary.

If you need to move from an August start to September, recruiting may be able to place you into another start group, but they may also tell you the original date is fixed.

Contact recruiting as early as possible and ask whether another approved start date is available.