diff --git a/brian2/codegen/runtime/numpy_rt/templates/spikegenerator.py_ b/brian2/codegen/runtime/numpy_rt/templates/spikegenerator.py_
index 63c16767c..ca9d17755 100644
--- a/brian2/codegen/runtime/numpy_rt/templates/spikegenerator.py_
+++ b/brian2/codegen/runtime/numpy_rt/templates/spikegenerator.py_
@@ -1,25 +1,48 @@
-{# USES_VARIABLES { _spikespace, neuron_index, _timebins, _period_bins, _lastindex, t_in_timesteps } #}
-{% extends 'common_group.py_' %}
+{% extends 'common_group.py.jinja2' %}
+
+{% block before_code %}
+# Copy of the SpikeGeneratorGroup.before_run code
+dt = {{dt.item()}}  # Always copy dt
+period = {{period_}}  # Always copy period
+
+# Always recalculate timesteps
+from brian2 import defaultclock  # Use Brian 2's clock instead of 't'
+current_t = defaultclock.t
+timesteps = ({{_spike_time}} / dt).astype(np.int32)
+current_step = int(current_t / dt)
+
+# Always update _lastindex
+in_the_past = np.nonzero(timesteps < current_step)[0]
+if len(in_the_past):
+    {{_lastindex}}[0] = in_the_past[-1] + 1
+else:
+    {{_lastindex}}[0] = 0
+
+# Always recalculate _timebins
+shift = 1e-3 * dt
+timebins = np.asarray(({{_spike_time}} + shift) / dt, dtype=np.int32)
+
+{{_timebins}}[:] = timebins
+
+# Always recalculate period_bins (ignore limit checks)
+period_bins = int(round(period / dt))
+{{_period_bins}}[0] = period_bins
+{% endblock %}
 
 {% block maincode %}
 _the_period    = {{_period_bins}}
-_timebin       = {{t_in_timesteps}}
+_timebin       = int(defaultclock.t / {{dt.item()}})  # Use Brian 2's clock instead of 't_in_timesteps'
 _n_spikes      = 0
 
 _lastindex_before = {{_lastindex}}
 
-if _the_period > 0:
-    _timebin %= _the_period
-    # If there is a periodicity in the SpikeGenerator, we need to reset the
-    # lastindex when the period has passed
-    if _lastindex_before > 0 and {{_timebins}}[_lastindex_before - 1] >= _timebin:
-        _lastindex_before = 0
+# Always reset _lastindex if period is applied
+_timebin %= _the_period
+_lastindex_before = 0
 
-_n_spikes = _numpy.searchsorted({{_timebins}}[_lastindex_before:],
-                                _timebin, side='right')
+_n_spikes = _numpy.searchsorted({{_timebins}}, _timebin, side='right')
 
 {{_lastindex}} = _lastindex_before + _n_spikes
-
 _indices = {{neuron_index}}[_lastindex_before:_lastindex_before+_n_spikes]
 
 {{_spikespace}}[:_n_spikes] = _indices
diff --git a/brian2/devices/cpp_standalone/templates/spikegenerator.cpp b/brian2/devices/cpp_standalone/templates/spikegenerator.cpp
index 691a55805..eeaba2580 100644
--- a/brian2/devices/cpp_standalone/templates/spikegenerator.cpp
+++ b/brian2/devices/cpp_standalone/templates/spikegenerator.cpp
@@ -1,23 +1,56 @@
-{# USES_VARIABLES { _spikespace, neuron_index, _timebins, _period_bins, _lastindex, t_in_timesteps, N } #}
+{# USES_VARIABLES { _spikespace, neuron_index, _timebins, _period_bins, _lastindex, N, _spike_time, dt, period } #}
 {% extends 'common_group.cpp' %}
+
+{% block before_code %}
+    // Copy of the SpikeGeneratorGroup.before_run code
+    const double _dt = {{dt.item()}};  // Always copy dt
+    const double _period = {{period_}};  // Always copy period
+
+    // Always recalculate _timesteps
+    std::vector<int32_t> _timesteps({{_spike_time}}.size());
+    for (size_t i = 0; i < _timesteps.size(); i++) {
+        _timesteps[i] = static_cast<int32_t>({{_spike_time}}[i] / _dt);
+    }
+
+    // Get current simulation time from Brian 2's clock instead of 't'
+    extern double defaultclock_t;  
+    const int32_t _current_step = static_cast<int32_t>(defaultclock_t / _dt);
+
+    // Always update _lastindex
+    int32_t _last_idx = 0;
+    for (size_t i = 0; i < _timesteps.size(); i++) {
+        if (_timesteps[i] < _current_step) {
+            _last_idx = i + 1;
+        } else {
+            break;
+        }
+    }
+    {{_lastindex}} = _last_idx;
+
+    // Always recalculate _timebins
+    const double _shift = 1e-3 * _dt;
+    std::vector<int32_t> _timebins({{_spike_time}}.size());
+    for (size_t i = 0; i < _timebins.size(); i++) {
+        _timebins[i] = static_cast<int32_t>({{_spike_time}}[i] + _shift) / _dt;
+    }
+    {{_timebins}} = _timebins;
+
+    // Always recalculate _period_bins 
+    {{_period_bins}} = static_cast<int32_t>(std::round(_period / _dt));
+{% endblock %}
+
 {% block maincode %}
 
     const int32_t _the_period = {{_period_bins}};
-    int32_t _timebin          = {{t_in_timesteps}};
-    const int32_t _n_spikes   = 0;
-
-    if (_the_period > 0) {
-        _timebin %= _the_period;
-        // If there is a periodicity in the SpikeGenerator, we need to reset the
-        // lastindex when the period has passed
-        if ({{_lastindex}} > 0 && {{_timebins}}[{{_lastindex}} - 1] >= _timebin)
-            {{_lastindex}} = 0;
-    }
+    int32_t _timebin = static_cast<int32_t>(defaultclock_t / {{dt.item()}});  // Use Brian 2's clock instead of 't_in_timesteps'
+
+    // Always recalculate timebin with period
+    _timebin %= _the_period;
+    {{_lastindex}} = 0;
 
     int32_t _cpp_numspikes = 0;
 
-    for(size_t _idx={{_lastindex}}; _idx < _num_timebins; _idx++)
-    {
+    for (size_t _idx = {{_lastindex}}; _idx < _num_timebins; _idx++) {
         if ({{_timebins}}[_idx] > _timebin)
             break;
 
@@ -28,5 +61,4 @@
 
     {{_lastindex}} += _cpp_numspikes;
 
-
-{% endblock %}
\ No newline at end of file
+{% endblock %}