ntp-dst

A fake NTP server for Mondaine SBB wall clocks (and similar dumb NTP clocks) that compensates for daylight saving time transitions. The clock applies a fixed UTC offset and doesn't know about DST — this server fakes the NTP time so the clock always shows the correct local time.

How it works

The Mondaine MSM.25S11 clock treats NTP time as UTC and adds a fixed offset configured during setup. It syncs only once per 24 hours to save battery. If configured as UTC+1 (winter/CET), it will always add +1h — even during summer.

This server serves UTC + dstCorrection where dstCorrection = correctLocalOffset - clockOffset. When a DST transition is detected, the correction is applied immediately. For a clock configured as UTC+1:

Season	Correct offset	Clock offset	DST correction	NTP serves	Clock displays
CET	UTC+1	+1h	0	UTC+0	UTC+0+1h = correct
CEST	UTC+2	+1h	+1h	UTC+1h	UTC+1h+1h = correct

Since the clock syncs only once per day, the correction takes effect the next time it syncs.

Quick Start

make build

# For a clock configured as UTC+1 (CET / winter time) — the default
sudo ./ntp-dst -port 123

# For a clock configured as UTC+2 (CEST / summer time)
sudo ./ntp-dst -port 123 -clock-offset 2h

# High port for testing (no root)
./ntp-dst -port 1234

Point your clock's NTP server (via DNS) to the machine running this server.

Configuration

Usage of ntp-dst:
  -port int          UDP port to listen on (default 123)
  -ntp string        Upstream NTP server pool (default "ch.pool.ntp.org")
  -clock-offset      Fixed UTC offset configured on the clock (default 1h)

Architecture

• main.go — Entry point, CLI flags
• scheduler.go — DST transition detection and correction
• source.go — Time source with NTP sync and DST correction
• server.go — UDP NTP server with query logging
• ntp.go — NTP packet marshaling/unmarshaling

Docker

docker build -t ntp-dst .
docker run -d --name ntp-dst \
  -p 123:123/udp \
  ntp-dst -clock-offset 1h

Unikernel (Nanos)

Run as a Nanos unikernel via OPS — no OS to maintain, no SSH, minimal attack surface.

# Install OPS
curl https://ops.city/get.sh -sSfL | sh

# x86_64 (VM.Standard.E2.1.Micro)
make build-unikernel
ops run -c ops.json ntp-dst

# ARM64 / Ampere A1 (VM.Standard.A1.Flex)
make build-unikernel-arm64
ops run -c ops.arm64.json ntp-dst-arm64

Deploy to OCI

Fill in BucketName and BucketNamespace in ops.json or ops.arm64.json, then:

# x86_64
ops image create ntp-dst -t oci -c ops.json
ops instance create ntp-dst -t oci -c ops.json

# ARM64
ops image create ntp-dst-arm64 -t oci -c ops.arm64.json --arch=arm64
ops instance create ntp-dst-arm64 -t oci -c ops.arm64.json

Remember to open UDP 123 in your OCI security list for the instance's VCN.

Running Tests

make test

Reference

• How smart is the Mondaine MSM.25S11 wifi wall clock? — The Mondaine clock uses an ESP chip, syncs once per 24h via NTP from pool.ntp.org, and applies a fixed timezone offset configured at setup time.