diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa16f99..2236dea 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Change Logs
 
+## 0.9.12 - 2020-05-30
+
+- show full datetime in output of `history -d`.
+- Fixed divide by zero panic in arithmetic (e.g. `2 / 0`).
+- Arithmetic commands change `previous status` too.
+
 ## 0.9.11 - 2020-04-26
 
 - Upgraded some deps.
diff --git a/Cargo.toml b/Cargo.toml
index 2013747..966c979 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,7 +2,7 @@
 edition = "2018"
 build = "src/build.rs"
 name = "cicada"
-version = "0.9.11"
+version = "0.9.12"
 authors = ["Hugo Wang <w@mitnk.com>"]
 
 description = "A simple Bash-like Unix shell."
diff --git a/docs/envs.md b/docs/envs.md
index 2014892..0de2894 100644
--- a/docs/envs.md
+++ b/docs/envs.md
@@ -52,14 +52,3 @@ default: `cicada_history`
 Do not exit cicada on `Ctrl-D`, if this env is set to `1`.
 
 default: `0`
-
-## Other Built-in Variables
-
-```
-$ ls file-not-exist
-$ echo $?  # <-- print exit status of previous command
-1
-
-$ echo $$  # <-- print PID of current process (cicada)
-2173
-```
diff --git a/docs/install.md b/docs/install.md
index ccff8f9..8e3a6fb 100644
--- a/docs/install.md
+++ b/docs/install.md
@@ -1,8 +1,8 @@
-## Install Cicada
+# Install Cicada
 
 There are a few ways to install cicada into your system.
 
-### 1) Install via Pre-built Binaries
+## 1) Install via Pre-built Binaries
 
 First download the latest right binary for your system from
 [Release Page](https://github.com/mitnk/cicada/releases).
@@ -23,9 +23,25 @@ $ cicada
 (in-cicada) $ cinfo
 ```
 
-You may want to [set cicada as the default shell](https://github.com/mitnk/cicada/blob/master/docs/install.md#user-content-set-cicada-as-your-login-shell).
+### Set cicada as your login shell
+
+**WARNING**: Please test cicada on your system before set it as default shell.
+
+In file `/etc/shells`, append the following line:
+
+```
+/usr/local/bin/cicada
+```
+
+Then run
+
+```
+$ chsh -s /usr/local/bin/cicada
+```
+
+Next time you open a terminal window, cicada shell will be the default one.
 
-### 2) Install via cargo crates
+## 2) Install via cargo crates
 
 If you already have [Rust environment](https://rustup.rs/), you can install
 cicada with `cargo`:
@@ -41,7 +57,7 @@ $ mv ~/.cargo/bin/cicada /usr/local/bin/
 $ cicada
 ```
 
-### 3) Install via Source
+## 3) Install via Source
 
 Note: [Rust environment](https://rustup.rs/) (Rust stable or above) is required.
 
@@ -55,20 +71,3 @@ $ make install
 $ cicada
 ```
 
-### Set cicada as your login shell
-
-**WARNING**: Please test cicada on your system before set it as default shell.
-
-In file `/etc/shells`, append the following line:
-
-```
-/usr/local/bin/cicada
-```
-
-Then run
-
-```
-$ chsh -s /usr/local/bin/cicada
-```
-
-Next time you open a terminal window, cicada shell will be the default one.
diff --git a/docs/rc-file.md b/docs/rc-file.md
index 773f6bd..256a3bb 100644
--- a/docs/rc-file.md
+++ b/docs/rc-file.md
@@ -16,7 +16,10 @@ first existing file of below list:
 Here is a sample RC file:
 
 ```
-# handle envs
+# customize my prompt on Mac
+export PROMPT="${GITBR}${COLOR_STATUS} $CWD${RESET}$ "
+
+# handle some envs
 export RUST_BACKTRACE='full'
 export COPYFILE_DISABLE=1
 export PATH="/usr/local/bin:$PATH"
@@ -26,6 +29,11 @@ alias ls="ls -G"
 alias ll="ls -lh"
 alias foo='echo foo bar | wc'
 
+# define functions
+function foo-bar() {
+    echo foobar
+}
+
 # run regular commands
 echo "cicada started at `date`" >> /tmp/some-random.log
 touch /tmp/cicada-started
diff --git a/src/builtins/history.rs b/src/builtins/history.rs
index 269b6c0..d150c3b 100644
--- a/src/builtins/history.rs
+++ b/src/builtins/history.rs
@@ -145,7 +145,7 @@ fn list_current_history(sh: &shell::Shell, conn: &Conn, opt: &OptMain) -> i32 {
                             }
                         };
                         let dt = NaiveDateTime::from_timestamp(tsb as i64, 0);
-                        println!("{}: {}: {}", row_id, dt.date(), inp);
+                        println!("{}: {}: {}", row_id, dt.format("%Y-%m-%d %H:%M:%S"), inp);
                     } else {
                         println!("{}: {}", row_id, inp);
                     }
diff --git a/src/calculator/mod.rs b/src/calculator/mod.rs
index cb69580..b46ef5c 100644
--- a/src/calculator/mod.rs
+++ b/src/calculator/mod.rs
@@ -31,7 +31,13 @@ pub fn eval_int(expression: Pairs<Rule>) -> i64 {
             Rule::add      => lhs + rhs,
             Rule::subtract => lhs - rhs,
             Rule::multiply => lhs * rhs,
-            Rule::divide   => lhs / rhs,
+            Rule::divide   => {
+                if rhs == 0 {
+                    (lhs as f64 / 0.0) as i64
+                } else {
+                    lhs / rhs
+                }
+            }
             Rule::power    => lhs.pow(rhs as u32),
             _ => unreachable!(),
         },
diff --git a/src/execute.rs b/src/execute.rs
index 5cb65ef..f18c23e 100644
--- a/src/execute.rs
+++ b/src/execute.rs
@@ -43,6 +43,7 @@ pub fn run_procs(sh: &mut shell::Shell,
                 } else {
                     println!("{}", result);
                 }
+                sh.previous_status = cr.status;
                 cr_list.push(cr);
                 return cr_list;
             }
@@ -53,6 +54,7 @@ pub fn run_procs(sh: &mut shell::Shell,
                 } else {
                     println_stderr!("cicada: calculator: {}", e);
                 }
+                sh.previous_status = cr.status;
                 cr_list.push(cr);
                 return cr_list;
             }