diff --git a/rtl/ptp_td_phc.v b/rtl/ptp_td_phc.v
index 9c84d4a82..0f3f1357c 100644
--- a/rtl/ptp_td_phc.v
+++ b/rtl/ptp_td_phc.v
@@ -454,13 +454,13 @@ always @(posedge clk) begin
                 adder_b_reg <= -NS_PER_S;
                 adder_cin_reg <= 0;
 
-                if (ts_tod_ns_reg[29]) begin
-                    // latter half of second; compute offset for next second
+                if (ts_tod_ns_reg[29:27] = 3'b111) begin
+                    // latter portion of second; compute offset for next second
                     adder_b_reg <= -NS_PER_S;
                     update_state_reg <= 12;
                     adder_busy_reg <= 1'b1;
                 end else begin
-                    // former half of second; compute offset for previous second
+                    // former portion of second; compute offset for previous second
                     adder_b_reg <= NS_PER_S;
                     update_state_reg <= 14;
                     adder_busy_reg <= 1'b1;
diff --git a/tb/ptp_td.py b/tb/ptp_td.py
index 635b7b994..cffa914b6 100644
--- a/tb/ptp_td.py
+++ b/tb/ptp_td.py
@@ -255,12 +255,12 @@ async def _run(self):
             self.ts_tod_offset_ns = (self.ts_tod_ns - self.ts_rel_ns) & 0xffffffff
 
             # compute alternate offset
-            if self.ts_tod_ns & (1 << 29):
-                # latter half of second; compute offset for next second
+            if self.ts_tod_ns >> 27 == 7:
+                # latter portion of second; compute offset for next second
                 self.ts_tod_alt_s = self.ts_tod_s+1
                 self.ts_tod_alt_offset_ns = (self.ts_tod_offset_ns - 1000000000) & 0xffffffff
             else:
-                # former half of second; compute offset for previous second
+                # former portion of second; compute offset for previous second
                 self.ts_tod_alt_s = self.ts_tod_s-1
                 self.ts_tod_alt_offset_ns = (self.ts_tod_offset_ns + 1000000000) & 0xffffffff