error[E0277]: `fn(Timer) {run_timer}` does not describe a valid system configuration
   --> src/main.rs:9:34
    |
9   |   App::new().add_systems(Update, run_timer).run();
    |              -----------         ^^^^^^^^^ invalid system configuration
    |              |
    |              required by a bound introduced by this call
    |
    = help: the trait `IntoSystem<(), (), _>` is not implemented for fn item `fn(Timer) {run_timer}`
    = help: the following other types implement trait `IntoSystemConfigs<Marker>`:
              `(S0, S1)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1)>`
              `(S0, S1, S2)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2)>`
              `(S0, S1, S2, S3)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3)>`
              `(S0, S1, S2, S3, S4)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4)>`
              `(S0, S1, S2, S3, S4, S5)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5)>`
              `(S0, S1, S2, S3, S4, S5, S6)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6)>`
              `(S0, S1, S2, S3, S4, S5, S6, S7)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6, P7)>`
              `(S0, S1, S2, S3, S4, S5, S6, S7, S8)` implements `IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6, P7, P8)>`
            and 14 others
    = note: required for `fn(Timer) {run_timer}` to implement `IntoSystemConfigs<_>`
note: required by a bound in `bevy::prelude::App::add_systems`
   --> /Users/gavinleroy/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bevy_app-0.14.0/src/app.rs:287:23
    |
284 |     pub fn add_systems<M>(
    |            ----------- required by a bound in this associated function
...
287 |         systems: impl IntoSystemConfigs<M>,
    |                       ^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::add_systems`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `simple-bevy-game` (bin "simple-bevy-game") due to 1 previous error