diff --git a/activemodel/lib/active_model/type/helpers/time_value.rb b/activemodel/lib/active_model/type/helpers/time_value.rb
index cb6aa67a9df8538b9bae6686e2f46fd2bf424eb1..da560734368414a8989164b0876990d5cedfbd93 100644
--- a/activemodel/lib/active_model/type/helpers/time_value.rb
+++ b/activemodel/lib/active_model/type/helpers/time_value.rb
@@ -70,7 +70,13 @@ def new_time(year, mon, mday, hour, min, sec, microsec, offset = nil)
           # Doesn't handle time zones.
           def fast_string_to_time(string)
             if string =~ ISO_DATETIME
-              microsec = ($7.to_r * 1_000_000).to_i
+              microsec_part = $7
+              if microsec_part && microsec_part.start_with?(".") && microsec_part.length == 7
+                microsec_part[0] = ""
+                microsec = microsec_part.to_i
+              else
+                microsec = (microsec_part.to_r * 1_000_000).to_i
+              end
               new_time $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, microsec
             end
           end