Treat backend's CRONS as UTC

common/time/consts.ts

@@ -1 +1,2 @@
 export const dayInMs = 1000 * 60 * 60 * 24;
+export const dayInMins = 60 * 24;

common/time/cron.ts

@@ -1,16 +1,43 @@
 import { type CronExpression, parseExpression } from "cron-parser";
 
+import { dayInMins } from "./consts";
 import { formatShortOffset } from "./offset";
+import { getTzMinuteDiff } from "./timezone";
 
 import type { Translator } from "@/common/i18n";
+import { parseNum } from "@/common/parsing";
 
 export class Cron {
-  private readonly expression: CronExpression;
+  public static fromUtcString(utcCron: string): Cron | null {
+    const utcParts = utcCron.split(" ");
+    const utcMin = parseNum(utcParts[0] ?? "");
+    const utcHour = parseNum(utcParts[1] ?? "");
 
-  public constructor(cron: string) {
-    this.expression = parseExpression(cron);
+    if (utcMin === null || utcHour === null) {
+      return null;
+    }
+
+    const utcTotalMins = utcHour * 60 + utcMin;
+    const localTotalMins = mod(utcTotalMins - getTzMinuteDiff(), dayInMins);
+    const localHour = Math.floor(localTotalMins / 60);
+    const localMin = localTotalMins % 60;
+
+    return Cron.fromLocalString(`${localMin} ${localHour} * * *`);
+  }
+
+  public static fromLocalString(localCron: string): Cron | null {
+    try {
+      return new Cron(parseExpression(localCron), localCron);
+    } catch {
+      return null;
+    }
   }
 
+  private constructor(
+    private readonly expression: CronExpression,
+    private readonly source: string,
+  ) {}
+
   public nextDate(): Date {
     const next = this.expression.next();
     this.expression.reset();
@@ -40,3 +67,5 @@ export class Cron {
     return formatShortOffset(tillPrev < tillNext ? prevDate : nextDate, t);
   }
 }
+
+const mod = (a: number, b: number): number => ((a % b) + b) % b;

common/time/index.ts

@@ -1,3 +1,4 @@
 export { Cron } from "./cron";
 export { formatShortOffset, formatLongOffset } from "./offset";
 export { toMinutesAndSeconds } from "./timer";
+export { getTzMinuteDiff } from "./timezone";

common/time/timezone.ts

@@ -0,0 +1 @@
+export const getTzMinuteDiff = () => new Date().getTimezoneOffset();

logic/medication/reminder.ts

@@ -36,7 +36,7 @@ export class Reminder {
       data.amountPerIntake,
       data.amountOwned ?? null,
       data.amountUnit ?? null,
-      data.cron ? new Cron(data.cron) : null,
+      data.cron ? Cron.fromUtcString(data.cron) : null,
       data.description ?? null,
     );
   }