diff --git a/api/api.go b/api/api.go
index fb03f90..25cbf59 100644
--- a/api/api.go
+++ b/api/api.go
@@ -404,6 +404,10 @@
 		return fmt.Errorf("Rescheduling UpdateBooking: Could not parse HTTP response to pb3: %v", err)
 	}
 
+	// Update slot before performing diff.
+	newBooking.GetSlot().StartSec = lastAvailability.GetStartSec()
+	newBooking.GetSlot().DurationSec = lastAvailability.GetDurationSec()
+
 	if iE := utils.ValidateBooking(resp.GetBooking(), newBooking); iE != nil {
 		return fmt.Errorf("invalid response. UpdateBooking: %s, abandoning slot 1/1", iE.Error())
 	}