To be honest, I think that if this is for final users (e.g., non-admin users), it won't be a very good idea. Making users distinguish between relative, absolute (as Vitaly mentioned), or ranges will come with a high cognitive load, and, in the end, it will always be a range.
Consider what Tableau does
As you can see, they use relative dates for admins, but users won't see that; they'll actually see a range.
Users shouldn't need to calculate dates, and relative dates are exactly that: a calculation. Hence, Tableau lets high-level admins choose that relative date, but final users see a range, as it should be.
Thus, just as there shouldn't be a "Relative Date," there shouldn't be an "Absolute date." Instead, the most common approach is to use "Date" or "Date Range" with pre-defined ranges or calendars where you can choose the date range. As an example of what I say, see how Google Analytics does it:
If this is for high-end admins, then you can use the "Relative Date" label, but for an absolute date, I think the proper way is "Custom Date" (or range).