I need to know the last date that I have to reverse a conversion from an IRA to a Roth IRA. My accountant has advised me that my AGI may be more than $100K for 1998. I have converted one IRA to a Roth IRA. So if, in fact, my AGI is in excess of $100K for this year I will need to reverse my conversion to the Roth IRA. My accountant thought that I probably had until 4/15/99 to make the reversal, but he wasn't sure.

You have until the EXTENDED due date. That would be 4/15/99 if you file by the normal due date, but could be as late as 10/15/99 if you file for extensions.