diff --git a/app/models/balance/base_calculator.rb b/app/models/balance/base_calculator.rb
index c8b0905479..0e4b5d24a8 100644
--- a/app/models/balance/base_calculator.rb
+++ b/app/models/balance/base_calculator.rb
@@ -9,6 +9,32 @@ def calculate
     raise NotImplementedError, "Subclasses must implement this method"
   end
 
+  # The earliest date balances should be materialized for.
+  #
+  # Normally this is the opening anchor date, which our system keeps at or
+  # before the oldest entry. But an entry (most commonly a backfilled
+  # reconciliation Valuation) can be created with a date EARLIER than the
+  # opening anchor — the reconciliation path does not move the anchor back.
+  # When that happens the anchor date alone would clip all pre-anchor entries
+  # out of the balance series (they'd be stored but never materialized into
+  # Balance rows, leaving the net-worth chart empty before the anchor).
+  #
+  # Bounding on min(opening_anchor_date, oldest_entry_date) ensures those
+  # earlier entries are included. The opening anchor and any reconciliation
+  # still reset the absolute balance on their own dates via the
+  # valuation-override path, so extending the window backward is safe.
+  #
+  # Public so Balance::Materializer can use the same lower bound when deciding
+  # which balances to preserve during an incremental purge.
+  #
+  # Memoized: this is read multiple times per sync (calc bound + both purge
+  # branches) and the underlying MIN(date) is a non-trivial scan on accounts
+  # with large entry histories. Calculator instances are per-sync, so there is
+  # no staleness concern.
+  def calculation_start_date
+    @calculation_start_date ||= [ account.opening_anchor_date, account.entries.minimum(:date) ].compact.min
+  end
+
   private
     def sync_cache
       @sync_cache ||= Balance::SyncCache.new(account)
diff --git a/app/models/balance/forward_calculator.rb b/app/models/balance/forward_calculator.rb
index 3e9956f605..29163b7c6e 100644
--- a/app/models/balance/forward_calculator.rb
+++ b/app/models/balance/forward_calculator.rb
@@ -108,6 +108,14 @@ def multi_currency_account?
     end
 
     def opening_starting_balances
+      # When the window starts before the opening anchor (a reconciliation or
+      # other entry was backfilled with a date earlier than the anchor), we have
+      # no known balance for that earlier start date. Seed flat at zero — the
+      # earliest reconciliation and the opening anchor each reset the absolute
+      # balance on their own dates via the valuation-override path, so the seed
+      # only affects the pre-anchor opening day's adjustment, not later totals.
+      return [ 0, 0 ] if calculation_start_date < account.opening_anchor_date
+
       cash = derive_cash_balance_on_date_from_total(
         total_balance: account.opening_anchor_balance,
         date: account.opening_anchor_date
@@ -123,7 +131,7 @@ def prior_balance
     end
 
     def calc_start_date
-      incremental? ? @window_start_date : account.opening_anchor_date
+      incremental? ? @window_start_date : calculation_start_date
     end
 
     def calc_end_date
diff --git a/app/models/balance/materializer.rb b/app/models/balance/materializer.rb
index 4a2066454c..59df779e68 100644
--- a/app/models/balance/materializer.rb
+++ b/app/models/balance/materializer.rb
@@ -80,10 +80,10 @@ def purge_stale_balances
         # In incremental forward-sync, even when no balances were calculated for the window
         # (e.g. window_start_date is beyond the last entry), purge stale tail records that
         # now fall beyond the prior-balance boundary so orphaned future rows are cleaned up.
-        if strategy == :forward && calculator.incremental? && account.opening_anchor_date <= @window_start_date - 1
+        if strategy == :forward && calculator.incremental? && calculator.calculation_start_date <= @window_start_date - 1
           deleted_count = account.balances.delete_by(
             "date < ? OR date > ?",
-            account.opening_anchor_date,
+            calculator.calculation_start_date,
             @window_start_date - 1
           )
           Rails.logger.info("Purged #{deleted_count} stale balances") if deleted_count > 0
@@ -95,11 +95,13 @@ def purge_stale_balances
 
       # In incremental forward-sync mode the calculator only recalculates from
       # window_start_date onward, so balances before that date are still valid.
-      # Use opening_anchor_date as the lower purge bound to preserve them.
+      # Use calculation_start_date as the lower purge bound to preserve them —
+      # this is the same lower bound the calculator uses, so pre-anchor balances
+      # (from entries dated before the opening anchor) are not deleted.
       # We ask the calculator whether it actually ran incrementally — it may have
       # fallen back to a full recalculation, in which case we use the normal bound.
       oldest_valid_date = if strategy == :forward && calculator.incremental?
-        account.opening_anchor_date
+        calculator.calculation_start_date
       else
         sorted_balances.first.date
       end
diff --git a/app/models/balance/reverse_calculator.rb b/app/models/balance/reverse_calculator.rb
index ba970fdcda..d073beda74 100644
--- a/app/models/balance/reverse_calculator.rb
+++ b/app/models/balance/reverse_calculator.rb
@@ -9,8 +9,13 @@ def calculate
       )
       end_non_cash_balance = account.current_anchor_balance - end_cash_balance
 
-      # Calculates in reverse-chronological order (End of day -> Start of day)
-      account.current_anchor_date.downto(account.opening_anchor_date).map do |date|
+      # Calculates in reverse-chronological order (End of day -> Start of day).
+      # Bound on calculation_start_date (not opening_anchor_date) so entries
+      # backfilled with a date earlier than the opening anchor are still
+      # materialized. Reconciliation waypoints below the anchor reset the
+      # balance on their own dates; use_opening_anchor_for_date? still keys off
+      # the anchor's real date, so the anchor's own treatment is unchanged.
+      account.current_anchor_date.downto(calculation_start_date).map do |date|
         flows = flows_for_date(date)
         valuation = sync_cache.get_valuation(date)
 
diff --git a/test/models/balance/forward_calculator_test.rb b/test/models/balance/forward_calculator_test.rb
index 22c8a5a616..b4f30c7c5b 100644
--- a/test/models/balance/forward_calculator_test.rb
+++ b/test/models/balance/forward_calculator_test.rb
@@ -757,6 +757,72 @@ class Balance::ForwardCalculatorTest < ActiveSupport::TestCase
     assert_not calculator.incremental?, "Should not be incremental for foreign currency accounts"
   end
 
+  # Regression: a reconciliation (or any entry) backfilled with a date EARLIER
+  # than the opening anchor must still be materialized into Balance rows. The
+  # window is bounded on min(opening_anchor_date, oldest_entry_date), and the
+  # pre-anchor start seeds from zero — each valuation resets the absolute
+  # balance on its own date.
+  test "materializes entries dated before the opening anchor" do
+    account = create_account_with_ledger(
+      account: { type: Depository, currency: "USD" },
+      entries: [
+        { type: "reconciliation", date: 5.days.ago.to_date, balance: 10000 }, # before anchor
+        { type: "opening_anchor",  date: 3.days.ago.to_date, balance: 17000 },
+        { type: "reconciliation", date: 1.day.ago.to_date,  balance: 20000 }
+      ]
+    )
+
+    calculated = Balance::ForwardCalculator.new(account).calculate
+
+    # Window extends back to the earliest entry, not the (later) opening anchor.
+    assert_equal 5.days.ago.to_date, calculated.map(&:date).min
+
+    assert_calculated_ledger_balances(
+      calculated_data: calculated,
+      expected_data: [
+        {
+          # Pre-anchor reconciliation: seeded from 0, valuation sets the balance.
+          date: 5.days.ago.to_date,
+          legacy_balances: { balance: 10000, cash_balance: 10000 },
+          balances: { start: 0, start_cash: 0, start_non_cash: 0, end_cash: 10000, end_non_cash: 0, end: 10000 },
+          flows: 0,
+          adjustments: { cash_adjustments: 10000, non_cash_adjustments: 0 }
+        },
+        {
+          # Carries forward on a day with no entries.
+          date: 4.days.ago.to_date,
+          legacy_balances: { balance: 10000, cash_balance: 10000 },
+          balances: { start: 10000, start_cash: 10000, start_non_cash: 0, end_cash: 10000, end_non_cash: 0, end: 10000 },
+          flows: 0,
+          adjustments: 0
+        },
+        {
+          # Opening anchor resets the balance on its own date.
+          date: 3.days.ago.to_date,
+          legacy_balances: { balance: 17000, cash_balance: 17000 },
+          balances: { start: 10000, start_cash: 10000, start_non_cash: 0, end_cash: 17000, end_non_cash: 0, end: 17000 },
+          flows: 0,
+          adjustments: { cash_adjustments: 7000, non_cash_adjustments: 0 }
+        },
+        {
+          # Carries forward on a day with no entries.
+          date: 2.days.ago.to_date,
+          legacy_balances: { balance: 17000, cash_balance: 17000 },
+          balances: { start: 17000, start_cash: 17000, start_non_cash: 0, end_cash: 17000, end_non_cash: 0, end: 17000 },
+          flows: 0,
+          adjustments: 0
+        },
+        {
+          date: 1.day.ago.to_date,
+          legacy_balances: { balance: 20000, cash_balance: 20000 },
+          balances: { start: 17000, start_cash: 17000, start_non_cash: 0, end_cash: 20000, end_non_cash: 0, end: 20000 },
+          flows: 0,
+          adjustments: { cash_adjustments: 3000, non_cash_adjustments: 0 }
+        }
+      ]
+    )
+  end
+
   private
     def assert_balances(calculated_data:, expected_balances:)
       # Sort calculated data by date to ensure consistent ordering
diff --git a/test/models/balance/materializer_test.rb b/test/models/balance/materializer_test.rb
index 472f5fbd51..6ee91d99db 100644
--- a/test/models/balance/materializer_test.rb
+++ b/test/models/balance/materializer_test.rb
@@ -116,6 +116,46 @@ class Balance::MaterializerTest < ActiveSupport::TestCase
       "Recalculated balance for 2.days.ago should be persisted"
   end
 
+  # Regression: a prior full sync materializes balances for entries dated BEFORE
+  # the opening anchor; a later incremental sync (window after the anchor) must
+  # preserve them. The purge lower bound uses calculation_start_date — the same
+  # bound the calculator uses — not opening_anchor_date.
+  test "incremental sync preserves balances dated before the opening anchor" do
+    # Opening anchor at 5.days.ago, but a reconciliation backfilled earlier.
+    @account.entries.create!(
+      name: "Opening Balance", date: 5.days.ago.to_date, amount: 5000,
+      currency: "USD", entryable: Valuation.new(kind: "opening_anchor")
+    )
+    @account.entries.create!(
+      name: "Backfilled reconciliation", date: 8.days.ago.to_date, amount: 3000,
+      currency: "USD", entryable: Valuation.new(kind: "reconciliation")
+    )
+
+    # Pre-anchor balance materialized by a prior full sync.
+    pre_anchor    = create_balance(account: @account, date: 8.days.ago.to_date, balance: 3000)
+    preserved_mid = create_balance(account: @account, date: 6.days.ago.to_date, balance: 4000)
+
+    recalculated = [
+      Balance.new(
+        date: 2.days.ago.to_date, balance: 15000, cash_balance: 15000, currency: "USD",
+        start_cash_balance: 12000, start_non_cash_balance: 0,
+        cash_inflows: 3000, cash_outflows: 0, non_cash_inflows: 0, non_cash_outflows: 0,
+        net_market_flows: 0, cash_adjustments: 0, non_cash_adjustments: 0, flows_factor: 1
+      )
+    ]
+
+    Balance::ForwardCalculator.any_instance.expects(:calculate).returns(recalculated)
+    Balance::ForwardCalculator.any_instance.stubs(:incremental?).returns(true)
+    Holding::Materializer.any_instance.expects(:materialize_holdings).returns([]).once
+
+    Balance::Materializer.new(@account, strategy: :forward, window_start_date: 2.days.ago.to_date).materialize_balances
+
+    assert_not_nil @account.balances.find_by(id: pre_anchor.id),
+      "Balance before the opening anchor must be preserved during incremental purge"
+    assert_not_nil @account.balances.find_by(id: preserved_mid.id),
+      "Balance between anchor and window must be preserved"
+  end
+
   test "falls back to full recalculation when window_start_date is given but no prior balance exists" do
     @account.entries.create!(
       name: "Opening Balance",
diff --git a/test/models/balance/reverse_calculator_test.rb b/test/models/balance/reverse_calculator_test.rb
index 11f4463e36..070e1f0ff0 100644
--- a/test/models/balance/reverse_calculator_test.rb
+++ b/test/models/balance/reverse_calculator_test.rb
@@ -686,4 +686,65 @@ class Balance::ReverseCalculatorTest < ActiveSupport::TestCase
       ]
     )
   end
+
+  # Regression: a reconciliation waypoint dated EARLIER than the opening anchor
+  # must still be materialized. The reverse loop is bounded on
+  # min(opening_anchor_date, oldest_entry_date), so it continues past the anchor
+  # down to the earliest entry, which resets the balance on its own date.
+  test "materializes reconciliation waypoints dated before the opening anchor" do
+    account = create_account_with_ledger(
+      account: { type: Depository, balance: 20000, cash_balance: 20000, currency: "USD" },
+      entries: [
+        { type: "current_anchor", date: Date.current, balance: 20000 },
+        { type: "opening_anchor",  date: 2.days.ago,  balance: 15000 },
+        { type: "reconciliation", date: 4.days.ago,  balance: 9000 } # before the opening anchor
+      ]
+    )
+
+    calculated = Balance::ReverseCalculator.new(account).calculate
+
+    # Loop extends back to the earliest entry, not the (later) opening anchor.
+    assert_equal 4.days.ago.to_date, calculated.map(&:date).min
+
+    assert_calculated_ledger_balances(
+      calculated_data: calculated,
+      expected_data: [
+        {
+          date: Date.current,
+          legacy_balances: { balance: 20000, cash_balance: 20000 },
+          balances: { start: 20000, start_cash: 20000, start_non_cash: 0, end_cash: 20000, end_non_cash: 0, end: 20000 },
+          flows: 0,
+          adjustments: 0
+        }, # Current anchor
+        {
+          date: 1.day.ago,
+          legacy_balances: { balance: 20000, cash_balance: 20000 },
+          balances: { start: 20000, start_cash: 20000, start_non_cash: 0, end_cash: 20000, end_non_cash: 0, end: 20000 },
+          flows: 0,
+          adjustments: 0
+        }, # Gap above the anchor carries down with no flows
+        {
+          date: 2.days.ago,
+          legacy_balances: { balance: 15000, cash_balance: 15000 },
+          balances: { start: 15000, start_cash: 15000, start_non_cash: 0, end_cash: 15000, end_non_cash: 0, end: 15000 },
+          flows: 0,
+          adjustments: 0
+        }, # Opening anchor
+        {
+          date: 3.days.ago,
+          legacy_balances: { balance: 15000, cash_balance: 15000 },
+          balances: { start: 15000, start_cash: 15000, start_non_cash: 0, end_cash: 15000, end_non_cash: 0, end: 15000 },
+          flows: 0,
+          adjustments: 0
+        }, # Gap below the anchor carries down with no flows
+        {
+          date: 4.days.ago,
+          legacy_balances: { balance: 9000, cash_balance: 9000 },
+          balances: { start: 9000, start_cash: 9000, start_non_cash: 0, end_cash: 9000, end_non_cash: 0, end: 9000 },
+          flows: 0,
+          adjustments: 0
+        } # Pre-anchor reconciliation resets to its own value
+      ]
+    )
+  end
 end