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app.py

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+"""
+Per Diem / Leave Start Date Calculator (Form e49)
+
+Reserve members accrue leave (typically 15 days for a 6-month deployment)
+and take it during the last 15 days of the deployment, so the last day of
+leave equals the end date on their orders. Given that end date, this app
+tells them when their leave starts.
+
+Rule: leave is 15 days inclusive of start and end date.
+If the end date falls on a weekend or federal holiday, the member gets
+one extra day of leave (so the start date shifts one day earlier).
+"""
+
+import gradio as gr
+import holidays
+from datetime import date, datetime, timedelta
+
+US_HOLIDAYS = holidays.US()
+
+
+def is_holiday(end_date: date) -> bool:
+    """Return True if end_date is a US federal holiday."""
+    return end_date in US_HOLIDAYS
+
+
+def is_weekend(end_date: date) -> bool:
+    """Return True if end_date is a Saturday or Sunday."""
+    return end_date.weekday() >= 5
+
+
+def fifteen_days_prior(end_date: date, exception: bool) -> date:
+    """
+    Return the date 15 days (inclusive) before end_date.
+    If `exception` is True (end date is a weekend/holiday), the member
+    gets one extra day of leave, so we go back 15 days instead of 14.
+    """
+    if exception:
+        return end_date - timedelta(days=15)
+    return end_date - timedelta(days=14)
+
+
+def calc_leave_dates(end_date: date) -> tuple[date, date]:
+    """Determine the leave start date given the leave end date."""
+    exception = is_weekend(end_date) or is_holiday(end_date)
+    start_date = fifteen_days_prior(end_date, exception)
+    return start_date, end_date
+
+
+def submit_leave_date(end_of_leave):
+    if end_of_leave is None:
+        return "Please select a date."
+
+    # gr.DateTime can hand back either a datetime object or a string
+    # depending on how the value was set, so handle both defensively.
+    if isinstance(end_of_leave, str):
+        end_of_leave = datetime.strptime(
+            end_of_leave.split(" ")[0], "%Y-%m-%d"
+        ).date()
+    elif isinstance(end_of_leave, (int, float)):
+        end_of_leave = datetime.fromtimestamp(end_of_leave).date()
+    else:
+        end_of_leave = end_of_leave.date()
+
+    start_date, end_date = calc_leave_dates(end_of_leave)
+
+    reason = []
+    if is_weekend(end_date):
+        reason.append("your end date falls on a weekend")
+    if is_holiday(end_date):
+        reason.append("your end date is a federal holiday")
+    extra_note = f" (You get an extra day of leave because {' and '.join(reason)}.)" if reason else ""
+
+    return (
+        f"Your leave starts on {start_date:%B %d, %Y} "
+        f"and ends on {end_date:%B %d, %Y}.{extra_note}"
+    )
+
+
+with gr.Blocks(title="Leave Start Date Calculator") as app:
+    gr.Markdown(
+        "# Leave Start Date Calculator\n"
+        "Enter the **end date of your deployment (Form e49 end date)** "
+        "to find out when your 15 days of leave will start."
+    )
+    date_input = gr.DateTime(
+        label="When is the end of your leave?",
+        include_time=False,
+        type="datetime",
+    )
+    submit_btn = gr.Button("Calculate", variant="primary")
+    output = gr.Textbox(label="Result", interactive=False)
+
+    submit_btn.click(fn=submit_leave_date, inputs=date_input, outputs=output)
+    date_input.submit(fn=submit_leave_date, inputs=date_input, outputs=output)
+
+
+if __name__ == "__main__":
+    app.launch()

requirements.txt

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+gradio
+holidays