The short answer
Yes, you can still book, and a great many people do.
This is the most common objection people raise before a consultation, usually apologetically, as though an unknown birth time were a personal failing. It is not. Times were recorded inconsistently for decades, families move, records are lost, and outside a few countries nobody wrote it down at all.
What matters is not whether you know it. It is whether you say you do not know it.
What still works without it
More than people expect. Most of the method survives.
The planetary relationships. In Bhrigu Nandi Nadi the primary unit of analysis is the yuti, meaning which planets share a house, and what those combinations produce. The planets' positions relative to one another do not depend on the birth time at all for most of a day.
The hero planet and its condition. Venus in a male chart and Mars in a female chart for marriage; Saturn for profession; Mercury for business and learning. What sits with that planet, and therefore what modifies the story, is largely readable.
Transit activation. The timing layer, which is what BNN is best at, works from the slow planets moving through the chart against those relationships. Windows can still be identified.
The degree sequence. Lower degree acts earlier, higher degree later. Unaffected.
So the timing questions people most often arrive with, when marriage is supported, when a career move is favoured, when a difficult period lifts, remain substantially answerable.
What weakens
One thing, and it has consequences.
The birth time sets the rising sign, which advances through all twelve signs in roughly twenty-four hours, changing about every two hours. The rising sign decides where house one begins, and therefore which house every planet occupies.
Without it, two things get thinner:
Assigning patterns to specific life areas. A chain can be identified without knowing which house it sits in, but knowing whether it is speaking to career, home or partnership depends on the house framework. So the reading can often say "this is the pattern" more confidently than "this is the area".
The Moon's position, if the time is very uncertain. The Moon moves about thirteen degrees a day. Over a full day of uncertainty its sign can change, and it carries mind, mood and the domestic side of a chart.
Why birth time accuracy matters sets out the mechanism in detail.
Why a guess is worse than a gap
This is the part worth understanding, because the instinct runs the other way.
A stated unknown produces a reading that is narrower and openly says which parts are less certain. You know what you are getting and you can weight it accordingly.
A guess produces a reading that is broad, possibly about a different chart entirely, and delivered with exactly the same confidence as a correct one. There is nothing in the output to signal the problem. You cannot tell, the reader cannot tell, and every conclusion inherits the error silently.
That asymmetry is the whole argument. An honest gap is a known limitation. A guess is an unknown one.
Give a range instead
If your family remembers something approximate, that is genuinely useful, provided you present it as what it is.
"Between roughly 11am and 1pm" is workable. It narrows the possible rising signs to two or three, and where the reading is checked against your past events, some of those can often be ruled out.
"Midday" is not workable, because it is a range wearing the clothes of a fact and nobody will know to treat it cautiously.
Say the range. Say who told you and how confident they were. That context is used.
Where to look before you give up
In rough order of reliability:
The birth certificate. In the UK the time is recorded, and a full certificate shows it. Ordering one is straightforward and inexpensive.
Hospital records, where they still exist for the period.
A baby book, a card, or a contemporaneous note. Something written at the time rather than recalled later.
An older relative's memory anchored to something else: a shift ending, a news broadcast, a meal. Anchored memory is considerably better than a bare number.
Family memory, decades later, unanchored. This is the least reliable source and it is usually the most confidently given, which is exactly the problem.
About rectification
Birth time rectification means working backwards from dated life events to estimate the time.
It is legitimate and it is genuinely difficult. It needs enough well-dated events to constrain the answer, and enough discipline not to quietly fit the time to whatever conclusion was wanted. Done casually it produces a confident number with no basis, which is the same problem as a guess with extra steps.
It is not part of a standard consultation. Where a reading has a range and a good set of past dates, some narrowing sometimes happens naturally as part of the checking, but that is a side effect rather than the service.
What to do when you book
Tick the box. There is one on the booking form for exactly this, and it exists because the situation is common rather than exceptional.
Bring what you do have: the date, the place, any range you have been told, and the dates of two or three significant past events. Those event dates matter more when the time is uncertain, not less, because they are what allows the reading to be checked and sometimes narrowed.
Nothing about the price changes. What changes is that the scope is set out at the start rather than discovered halfway through, and you will be told plainly which parts of the reading are firmer than others.
How to prepare for a reading covers the rest of what to bring, and what actually happens in a consultation describes the session. When you are ready, a BNN birth chart reading works with a stated unknown perfectly well.