diff --git a/src/callbacks/post_process.jl b/src/callbacks/post_process.jl
index 42b85fd669..97a90fcc15 100644
--- a/src/callbacks/post_process.jl
+++ b/src/callbacks/post_process.jl
@@ -202,12 +202,12 @@ end
 
 @inline function backup_condition(cb::PostprocessCallback{Int}, integrator)
     return integrator.stats.naccept > 0 &&
-           round(integrator.stats.naccept / cb.interval) % cb.write_file_interval == 0
+           (integrator.stats.naccept ÷ cb.interval) % cb.write_file_interval == 0
 end
 
 @inline function backup_condition(cb::PostprocessCallback, integrator)
     return integrator.stats.naccept > 0 &&
-           round(Int, integrator.t / cb.interval) % cb.write_file_interval == 0
+           get_iter(cb.interval, integrator) % cb.write_file_interval == 0
 end
 
 # After the simulation has finished, this function is called to write the data to a JSON file
diff --git a/test/Project.toml b/test/Project.toml
index 583f367bc9..02d49c16d5 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -13,6 +13,7 @@ Printf = "de0858da-6303-5e67-8744-51eddeeeb8d7"
 QuadGK = "1fd47b50-473d-5c70-9696-f719f8f3bcdc"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+TrixiParticles = "66699cd8-9c01-4e9d-a059-b96c86d16b3a"
 TrixiTest = "0a316866-cbd0-4425-8bcb-08103b2c1f26"
 
 [compat]
diff --git a/test/callbacks/postprocess.jl b/test/callbacks/postprocess.jl
index ccf6ebc9b3..ca3213e38f 100644
--- a/test/callbacks/postprocess.jl
+++ b/test/callbacks/postprocess.jl
@@ -138,4 +138,80 @@
         └──────────────────────────────────────────────────────────────────────────────────────────────────┘"""
         @test repr("text/plain", callback) == show_box
     end
+
+    @testset verbose=true "backup_condition" begin
+        function example_function(system, data, t)
+            return t
+        end
+
+        # Helper to create a mock integrator for backup_condition tests
+        function make_integrator(; naccept, t, tspan)
+            return (; stats=(; naccept), t, sol=(; prob=(; tspan)))
+        end
+
+        # Integer interval
+        @testset "interval-based (Int)" begin
+            cb_interval = PostprocessCallback(; interval=10, write_file_interval=3,
+                                              example_function)
+            pp = cb_interval.affect!
+
+            # No write at naccept=0 (initialization)
+            integrator = make_integrator(naccept=0, t=0.0, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            # Write at 1st, 2nd trigger: no write (write_file_interval=3)
+            integrator = make_integrator(naccept=10, t=0.1, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            integrator = make_integrator(naccept=20, t=0.2, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            # Write at 3rd trigger (3 * interval = 30)
+            integrator = make_integrator(naccept=30, t=0.3, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == true
+
+            integrator = make_integrator(naccept=40, t=0.4, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            # Write every write_file_interval triggers, regardless of t_0
+            # (naccept resets after restart, so counting is relative to new start)
+            integrator = make_integrator(naccept=30, t=10.3, tspan=(10.0, 11.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == true
+        end
+
+        # Float (dt-based)
+        @testset "dt-based (Float)" begin
+            cb_dt = PostprocessCallback(; dt=0.1, write_file_interval=3, example_function)
+            pp = cb_dt.affect!.affect!
+
+            # No write at naccept=0 (initialization)
+            integrator = make_integrator(naccept=0, t=0.0, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            # 1st and 2nd trigger: no write
+            integrator = make_integrator(naccept=1, t=0.1, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            integrator = make_integrator(naccept=2, t=0.2, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            # 3rd trigger: write
+            integrator = make_integrator(naccept=3, t=0.3, tspan=(0.0, 1.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == true
+
+            # After restart at t_0=1.0: counting restarts from tspan[1]
+            integrator = make_integrator(naccept=0, t=1.0, tspan=(1.0, 2.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            integrator = make_integrator(naccept=1, t=1.1, tspan=(1.0, 2.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            integrator = make_integrator(naccept=2, t=1.2, tspan=(1.0, 2.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == false
+
+            # 3rd trigger after restart: write (not at t=1.2 as the old code would give)
+            integrator = make_integrator(naccept=3, t=1.3, tspan=(1.0, 2.0))
+            @test TrixiParticles.backup_condition(pp, integrator) == true
+        end
+    end
 end