Simplify default values of `Time.at` [Bug #18293]

time.c

@@ -2678,13 +2678,13 @@ get_scale(VALUE unit)
 }
 
 static VALUE
-time_s_at(rb_execution_context_t *ec, VALUE klass, VALUE time, VALUE subsec, VALUE unit, VALUE zone, VALUE nosubsec, VALUE nounit)
+time_s_at(rb_execution_context_t *ec, VALUE klass, VALUE time, VALUE subsec, VALUE unit, VALUE zone)
 {
     VALUE t;
     wideval_t timew;
 
-    if (!RTEST(nosubsec)) {
-        int scale = !RTEST(nounit) ? get_scale(unit) : 1000000;
+    if (!NIL_P(subsec)) {
+        int scale = get_scale(unit);
         time = num_exact(time);
         t = num_exact(subsec);
         timew = wadd(rb_time_magnify(v2w(time)), wmulquoll(v2w(t), TIME_SCALE, scale));

timev.rb

@@ -267,8 +267,8 @@ class Time
   # :include: doc/time/nsec.rdoc
   # :include: doc/time/in.rdoc
   #
-  def self.at(time, subsec = (nosubsec = true), unit = (nounit = true), in: nil)
-    __builtin.time_s_at(time, subsec, unit, __builtin.arg!(:in), nosubsec, nounit)
+  def self.at(time, subsec = nil, unit = :microsecond, in: nil)
+    __builtin.time_s_at(time, subsec, unit, __builtin.arg!(:in))
   end
 
   # Returns a new \Time object based the on given arguments.