fixup: Improvements on LSPSDateTime

- Implement From<Duration> for LSPSDateTime
- Avoid doing ambiguous timestamp types
- Add abs_diff function to use on client / service

Author: martinsaposnic

lightning-liquidity/src/lsps0/ser.rs

@@ -18,6 +18,7 @@ use crate::lsps2::msgs::{
 };
 use crate::prelude::{HashMap, String};
 
+use chrono::DateTime;
 use lightning::ln::msgs::{DecodeError, LightningError};
 use lightning::ln::wire;
 use lightning::util::ser::{LengthLimitedRead, LengthReadable, WithoutLength};
@@ -27,6 +28,7 @@ use bitcoin::secp256k1::PublicKey;
 use core::fmt::{self, Display};
 use core::str::FromStr;
 
+use core::time::Duration;
 #[cfg(feature = "std")]
 use std::time::{SystemTime, UNIX_EPOCH};
 
@@ -212,6 +214,17 @@ impl LSPSDateTime {
 			self.0.timestamp().try_into().expect("expiration to be ahead of unix epoch");
 		now_seconds_since_epoch > datetime_seconds_since_epoch
 	}
+
+	/// Returns the time in seconds since the unix epoch.
+	pub fn abs_diff(&self, other: &Self) -> u64 {
+		self.0.timestamp().abs_diff(other.0.timestamp())
+	}
+}
+
+impl From<Duration> for LSPSDateTime {
+	fn from(duration: Duration) -> Self {
+		Self(DateTime::UNIX_EPOCH + duration)
+	}
 }
 
 impl FromStr for LSPSDateTime {