From e7c41a64241d50a3a604660af56f2ddcccfee847 Mon Sep 17 00:00:00 2001
From: Paul Bender <pebender@gmail.com>
Date: Fri, 10 Oct 2025 13:17:53 -0700
Subject: [PATCH] Generate error when century bit is set in functions already
 reading the century bit

---
 src/ds323x/datetime.rs | 56 ++++++++++++++++++++++++------------------
 src/lib.rs             |  5 ++++
 tests/datetime.rs      |  6 -----
 3 files changed, 37 insertions(+), 30 deletions(-)

diff --git a/src/ds323x/datetime.rs b/src/ds323x/datetime.rs
index 1a48e4c..3491ea7 100644
--- a/src/ds323x/datetime.rs
+++ b/src/ds323x/datetime.rs
@@ -19,11 +19,13 @@ where
         let mut data = [0; 8];
         self.iface.read_data(&mut data)?;
 
-        let year = year_from_registers(
-            data[Register::MONTH as usize + 1],
-            data[Register::YEAR as usize + 1],
-        );
-        let month = packed_bcd_to_decimal(data[Register::MONTH as usize + 1] & !BitFlags::CENTURY);
+        let century = data[Register::MONTH as usize + 1] & BitFlags::CENTURY;
+        if century != 0 {
+            return Err(Error::InvalidDeviceCentury);
+        }
+
+        let year = 2000 + (packed_bcd_to_decimal(data[Register::YEAR as usize + 1]) as u16);
+        let month = packed_bcd_to_decimal(data[Register::MONTH as usize + 1]);
         let day = packed_bcd_to_decimal(data[Register::DOM as usize + 1]);
         let hour = hours_from_register(data[Register::HOURS as usize + 1]);
         let minute = packed_bcd_to_decimal(data[Register::MINUTES as usize + 1]);
@@ -91,15 +93,28 @@ where
 
     fn month(&mut self) -> Result<u8, Self::Error> {
         let data = self.iface.read_register(Register::MONTH)?;
-        let value = data & !BitFlags::CENTURY;
-        Ok(packed_bcd_to_decimal(value))
+
+        let century = data & BitFlags::CENTURY;
+        if century != 0 {
+            return Err(Error::InvalidDeviceCentury);
+        }
+
+        Ok(packed_bcd_to_decimal(data))
     }
 
     fn year(&mut self) -> Result<u16, Self::Error> {
         let mut data = [0; 3];
         data[0] = Register::MONTH;
         self.iface.read_data(&mut data)?;
-        Ok(year_from_registers(data[1], data[2]))
+
+        let century = data[1] & BitFlags::CENTURY;
+        if century != 0 {
+            return Err(Error::InvalidDeviceCentury);
+        }
+
+        let year = 2000 + (packed_bcd_to_decimal(data[2]) as u16);
+
+        Ok(year)
     }
 
     fn date(&mut self) -> Result<NaiveDate, Self::Error> {
@@ -108,12 +123,15 @@ where
         self.iface.read_data(&mut data)?;
 
         let offset = Register::DOM as usize;
-        let year = year_from_registers(
-            data[Register::MONTH as usize + 1 - offset],
-            data[Register::YEAR as usize + 1 - offset],
-        );
-        let month =
-            packed_bcd_to_decimal(data[Register::MONTH as usize + 1 - offset] & !BitFlags::CENTURY);
+
+        let century = data[Register::MONTH as usize + 1 - offset] & BitFlags::CENTURY;
+        if century != 0 {
+            return Err(Error::InvalidDeviceCentury);
+        }
+
+        let year =
+            2000 + (packed_bcd_to_decimal(data[Register::YEAR as usize + 1 - offset]) as u16);
+        let month = packed_bcd_to_decimal(data[Register::MONTH as usize + 1 - offset]);
         let day = packed_bcd_to_decimal(data[Register::DOM as usize + 1 - offset]);
         let date = NaiveDate::from_ymd_opt(year.into(), month.into(), day.into());
         some_or_invalid_error(date)
@@ -231,16 +249,6 @@ fn hours_from_register(data: u8) -> Hours {
     }
 }
 
-fn year_from_registers(month: u8, year: u8) -> u16 {
-    let century = month & BitFlags::CENTURY;
-    let year = packed_bcd_to_decimal(year);
-    if century != 0 {
-        2100 + u16::from(year)
-    } else {
-        2000 + u16::from(year)
-    }
-}
-
 fn is_24h_format(hours_data: u8) -> bool {
     hours_data & BitFlags::H24_H12 == 0
 }
diff --git a/src/lib.rs b/src/lib.rs
index 5efa490..f98afda 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -388,6 +388,11 @@ pub enum Error<E> {
     /// It was not possible to read a valid date and/or time.
     /// The device is probably missing initialization.
     InvalidDeviceState,
+    /// Device century is not the 20th century.
+    ///
+    /// The device does not produce valid dates for centuries
+    /// other than the 20th century.
+    InvalidDeviceCentury,
 }
 
 /// Square-wave output frequency
diff --git a/tests/datetime.rs b/tests/datetime.rs
index eb465c3..ec88bdf 100644
--- a/tests/datetime.rs
+++ b/tests/datetime.rs
@@ -185,12 +185,6 @@ mod month {
     get_param_test!(get, month, MONTH, 1, 1);
     read_set_param_test!(set, set_month, MONTH, 12, 0b0000_0010, 0b0001_0010);
     set_invalid_param_range_test!(invalid, set_month, 0, 13);
-
-    mod keeps_century {
-        use super::*;
-        get_param_test!(get, month, MONTH, 12, 0b1001_0010);
-        read_set_param_test!(set, set_month, MONTH, 12, 0b1000_0010, 0b1001_0010);
-    }
 }
 
 mod year {