diff --git a/src/modem.rs b/src/modem.rs
index 51da832..e019ca0 100644
--- a/src/modem.rs
+++ b/src/modem.rs
@@ -62,7 +62,7 @@ impl<UART: serial::Uart> Iterator for IterableRx<UART> {
                         break None
                     }
                     thread::sleep(Duration::from_millis(200));
-                    self.timeout = Some(timeout - now.elapsed());
+                    self.timeout = Some(timeout.saturating_sub(now.elapsed()));
                 }
             }
         }
@@ -140,7 +140,7 @@ impl<UART: serial::Uart> Modem<UART> {
         let match_text: String = contains.unwrap_or("\n".to_string());
 
         loop {
-            let rdln = self.read_line(start + timeout - Instant::now());
+            let rdln = self.read_line(timeout.saturating_sub(start.elapsed()));
             if let Ok(line) = rdln {
                 println!("Read {} bytes from serial ({})", line.len(), line);
                 response.push_str(&line);