Use SystemTime intead of Instant

Files of sender and receiver are combined, thus it is important to have
a timestamp that is relative to common anchor, UNIX_EPOCH in this case

src/lib.rs

-use std::time::Instant;
+use std::time::SystemTime;
 #[cfg(feature = "use-xlap")]
 use std::{
     convert::AsRef,
@@ -50,7 +50,7 @@ include!(concat!(env!("OUT_DIR"), "/xlap_generated.rs"));
 #[cfg(feature = "use-xlap")]
 struct Timestamp {
     c: u64,
-    t: Instant,
+    t: SystemTime,
 }
 
 #[cfg(feature = "use-xlap")]
@@ -65,9 +65,6 @@ pub struct TimestampTable {
     rows: [TimestampTableRow; TS_ROWS],
 }
 
-#[cfg(feature = "use-xlap")]
-static mut ZERO: Option<Instant> = None;
-
 #[macro_export]
 /// Update the clock and cycle value of a timestamp by setting them to en explicit value
 macro_rules! set_timestamp {
@@ -81,7 +78,7 @@ macro_rules! set_timestamp {
 
 #[cfg(feature = "use-xlap")]
 #[inline(always)]
-pub unsafe fn set_timestamp(kind: Kind, row: usize, id: TimestampId, (t, c): (Instant, u64)) {
+pub unsafe fn set_timestamp(kind: Kind, row: usize, id: TimestampId, (t, c): (SystemTime, u64)) {
     set_timestamp_clock(kind, row, id, t);
     set_timestamp_cycle(kind, row, id, c);
 }
@@ -99,7 +96,7 @@ macro_rules! set_timestamp_clock {
 
 #[cfg(feature = "use-xlap")]
 #[inline(always)]
-pub unsafe fn set_timestamp_clock(kind: Kind, row: usize, id: TimestampId, value: Instant) {
+pub unsafe fn set_timestamp_clock(kind: Kind, row: usize, id: TimestampId, value: SystemTime) {
     (*XLAP[kind as usize].load(Ordering::Acquire)).rows[row % TS_ROWS].timestamp[id as usize].t =
         value;
 }
@@ -137,7 +134,7 @@ macro_rules! timestamp_clock {
 #[inline(always)]
 pub unsafe fn timestamp_clock(kind: Kind, row: usize, id: TimestampId) {
     (*XLAP[kind as usize].load(Ordering::Acquire)).rows[row % TS_ROWS].timestamp[id as usize].t =
-        Instant::now();
+        SystemTime::now();
 }
 
 #[macro_export]
@@ -167,8 +164,8 @@ macro_rules! get_timestamp_clock {
 }
 
 #[inline(always)]
-pub fn get_timestamp_clock() -> Instant {
-    Instant::now()
+pub fn get_timestamp_clock() -> SystemTime {
+    SystemTime::now()
 }
 
 #[macro_export]
@@ -243,7 +240,6 @@ pub unsafe fn init() {
     #[cfg(debug_assertions)] {
         println!("Initialised XLAP with {} rows.", TS_ROWS)
     }
-    ZERO = Some(Instant::now());
     for ptr in XLAP.iter() {
         // Create a zeroed Timestamp table
         let table: TimestampTable = std::mem::zeroed();
@@ -298,7 +294,7 @@ impl TimestampTable {
             for (i, row) in table.rows.iter().enumerate().skip(1) {
                 write!(&mut file, "{}, {}", i, *kind as usize)?;
                 for timestamp in row.timestamp.iter() {
-                    let t = timestamp.t.saturating_duration_since(ZERO.unwrap());
+                    let t = timestamp.t.duration_since(SystemTime::UNIX_EPOCH).unwrap();
                     write!(&mut file, ", {}, {}", t.as_micros(), timestamp.c)?;
                 }
                 writeln!(&mut file)?;