Add cron parser for start time of a reminder

src/time_parser.rs

@@ -6,6 +6,8 @@ use std::{
 
 use chrono::{DateTime, Datelike, Timelike, Utc};
 use chrono_tz::Tz;
+use cron_parser::parse;
+use std::str::FromStr;
 use tokio::process::Command;
 
 use crate::consts::{LOCAL_TIMEZONE, PYTHON_LOCATION};
@@ -219,3 +221,7 @@ pub async fn natural_parser(time: &str, timezone: &str) -> Option<i64> {
         })
         .and_then(|inner| if inner < 0 { None } else { Some(inner) })
 }
+
+pub fn cron_next_timestamp(expr: &str, timezone: Tz) -> Option<i64> {
+    parse(expr, &Utc::now().with_timezone(&timezone)).ok().map(|next| next.timestamp() as i64)
+}