std/time: add the addSec method to Time

std/time/time.jule

@@ -166,6 +166,19 @@ impl Time {
 		ret absSeconds(self.abs())
 	}
 
+	// Adds d seconds to the time.
+	fn addSec(mut self, d: i64) {
+		// Check if the sum of self.sec and d overflows and handle it properly.
+		sum := self.sec + d
+		if (sum > self.sec) == (d > 0) {
+			self.sec = sum
+		} else if d > 0 {
+			self.sec = 1<<63 - 1
+		} else {
+			self.sec = -(1<<63 - 1)
+		}
+	}
+
 	// Computes the time zone in effect at time specification, returning the abbreviated
 	// name of the zone (such as "CET") and its offset in seconds east of UTC.
 	fn Zone(self): (name: str, offset: int) {