---
epic: 4
story: 4.2
title: "Time Math"
status: draft
---

## Epic 4 — Date, Time & Time Zones
**Goal:** Full date/time math, business day calculations, and timezone awareness.

### Story 4.2: Time Math

As a CalcPad user,
I want to add and subtract durations from times and calculate durations between times,
So that I can compute meeting end times, elapsed time, and scheduling math.

**Acceptance Criteria:**

**Given** the expression `3:35 am + 9 hours 20 minutes`
**When** the engine evaluates it
**Then** the result is `12:55 pm`

**Given** the expression `3:35 PM + 9 hours 20 minutes`
**When** the engine evaluates it
**Then** the result is `12:55 AM` (next day)
**And** the result indicates the day rollover if applicable

**Given** the expression `14:30 - 2 hours 45 minutes`
**When** the engine evaluates it in 24-hour mode
**Then** the result is `11:45`

**Given** the user's time format preference is 12-hour
**When** time results are displayed
**Then** they use AM/PM notation

**Given** the user's time format preference is 24-hour
**When** time results are displayed
**Then** they use 24-hour notation (e.g., `14:30` instead of `2:30 PM`)

**Given** the expression `9:00 AM to 5:30 PM`
**When** the engine evaluates it
**Then** the result is `8 hours 30 minutes`

**Given** the expression `11:00 PM to 2:00 AM`
**When** the engine evaluates it
**Then** it assumes the time crosses midnight
**And** the result is `3 hours`