Add symbolic tspan field to System struct

This commit addresses issue #847 by adding a symbolic time span (tspan) field
to the System struct for time-dependent systems. The tspan field allows users
to specify the time interval for simulations directly in the system definition,
which can be either numeric values or symbolic expressions involving parameters.

Changes:
- Added tspan field to System struct with type Union{Nothing, Tuple{Any, Any}}
- Updated System constructors to accept tspan parameter with default value nothing
- Added documentation for the new tspan field
- Added comprehensive tests for tspan functionality including numeric, symbolic, and nil cases
- Tests verify that tspan is preserved through system completion

The tspan field is automatically accessible via get_tspan() and has_tspan()
getter methods through the existing SYS_PROPS metaprogramming framework.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

src/systems/system.jl

@@ -190,6 +190,11 @@ struct System <: IntermediateDeprecationSystem
     """
     tstops::Vector{Any}
     """
+    The time span for time-dependent systems. This is a tuple of the start and end
+    time for the system, which can be symbolic expressions involving parameters.
+    """
+    tspan::Union{Nothing, Tuple{Any, Any}}
+    """
     The `TearingState` of the system post-simplification with `mtkcompile`.
     """
     tearing_state::Any
@@ -256,7 +261,7 @@ struct System <: IntermediateDeprecationSystem
             defaults, guesses, systems, initialization_eqs, continuous_events, discrete_events,
             connector_type, assertions = Dict{BasicSymbolic, String}(),
             metadata = MetadataT(), gui_metadata = nothing,
-            is_dde = false, tstops = [], tearing_state = nothing, namespacing = true,
+            is_dde = false, tstops = [], tspan = nothing, tearing_state = nothing, namespacing = true,
             complete = false, index_cache = nothing, ignored_connections = nothing,
             preface = nothing, parent = nothing, initializesystem = nothing,
             is_initializesystem = false, is_discrete = false, isscheduled = false,
@@ -296,7 +301,7 @@ struct System <: IntermediateDeprecationSystem
             observed, parameter_dependencies, var_to_name, name, description, defaults,
             guesses, systems, initialization_eqs, continuous_events, discrete_events,
             connector_type, assertions, metadata, gui_metadata, is_dde,
-            tstops, tearing_state, namespacing, complete, index_cache, ignored_connections,
+            tstops, tspan, tearing_state, namespacing, complete, index_cache, ignored_connections,
             preface, parent, initializesystem, is_initializesystem, is_discrete,
             isscheduled, schedule)
     end
@@ -332,7 +337,7 @@ function System(eqs::Vector{Equation}, iv, dvs, ps, brownians = [];
         continuous_events = SymbolicContinuousCallback[], discrete_events = SymbolicDiscreteCallback[],
         connector_type = nothing, assertions = Dict{BasicSymbolic, String}(),
         metadata = MetadataT(), gui_metadata = nothing,
-        is_dde = nothing, tstops = [], tearing_state = nothing,
+        is_dde = nothing, tstops = [], tspan = nothing, tearing_state = nothing,
         ignored_connections = nothing, parent = nothing,
         description = "", name = nothing, discover_from_metadata = true,
         initializesystem = nothing, is_initializesystem = false, is_discrete = false,
@@ -417,7 +422,7 @@ function System(eqs::Vector{Equation}, iv, dvs, ps, brownians = [];
         costs, consolidate, dvs, ps, brownians, iv, observed, Equation[],
         var_to_name, name, description, defaults, guesses, systems, initialization_eqs,
         continuous_events, discrete_events, connector_type, assertions, metadata, gui_metadata, is_dde,
-        tstops, tearing_state, true, false, nothing, ignored_connections, preface, parent,
+        tstops, tspan, tearing_state, true, false, nothing, ignored_connections, preface, parent,
         initializesystem, is_initializesystem, is_discrete; checks)
 end
 

test/odesystem.jl

@@ -1614,3 +1614,31 @@ end
     @test_nowarn push!(arr, sys)
     @test_nowarn TestWrapper(sys)
 end
+
+@testset "Symbolic tspan" begin
+    @variables x(t) y(t)
+    @parameters σ ρ β
+
+    # Test creating a system with tspan
+    eqs = [D(x) ~ σ * (y - x),
+           D(y) ~ x * (ρ - x) - y]
+    
+    # Test with numeric tspan
+    @named sys1 = System(eqs, t, [x, y], [σ, ρ, β]; tspan = (0.0, 10.0))
+    @test ModelingToolkit.get_tspan(sys1) == (0.0, 10.0)
+    @test ModelingToolkit.has_tspan(sys1) == true
+    
+    # Test with symbolic tspan
+    @parameters t0 tf
+    @named sys2 = System(eqs, t, [x, y], [σ, ρ, β]; tspan = (t0, tf))
+    @test isequal(ModelingToolkit.get_tspan(sys2), (t0, tf))
+    
+    # Test without tspan
+    @named sys3 = System(eqs, t, [x, y], [σ, ρ, β])
+    @test ModelingToolkit.get_tspan(sys3) === nothing
+    @test ModelingToolkit.has_tspan(sys3) == true  # Field exists but is nothing
+    
+    # Test that tspan is preserved through system completion
+    sys1_complete = complete(sys1)
+    @test ModelingToolkit.get_tspan(sys1_complete) == (0.0, 10.0)
+end