TimeSync

Keeps your Windows clock accurate, and tells you in plain words what is wrong when it isn't.

TimeSync asks several time servers at once, takes the median, ignores outliers and only sets the clock when the servers agree. A small background service keeps it that way: after Windows starts, every hour, after waking from sleep and after a network change. The window shows your PC's time next to the actual time and explains the usual suspects: dual boot with Linux, a wrong time zone, automatic daylight saving time switched off, a weak CMOS battery, a company network that blocks the time port.

🌐 Website: tech.burnersen.de/en/tools/timesync-en · 🇩🇪 Auf Deutsch: tech.burnersen.de/tools/timesync

The window speaks German and English and follows the Windows light or dark theme. What changed in version 2.0: CHANGELOG.md.

What's in the box

File	What it is
TimeSync.exe	The window. Runs as a normal user; asks for administrator rights (UAC) only when needed.
ntp-sync.exe	Command line tool and Windows service. Every command line of version 1.x still works.

Requirements: Windows 10 or 11, x64 or ARM64. The window uses the Microsoft Edge WebView2 runtime, which is part of current Windows.

Install

• Installer (recommended): download TimeSync-<version>-setup-x64.exe (or -arm64) from the latest release and run it. Leave "keep the clock in sync in the background" ticked.
• Portable: download the .zip, unpack it anywhere and start TimeSync.exe. Setting up the background service copies ntp-sync.exe to C:\Program Files\TimeSync, where only administrators can change it. To keep the window's preferences next to the program, create an empty file portable.txt beside TimeSync.exe.
• winget / Scoop: coming soon (manifests are in packaging/).

Windows SmartScreen may warn because the files are not yet digitally signed. Choose "More info" → "Run anyway". The source code is all here, and every release is built by GitHub Actions from this repository.

How it decides

Every synchronization round:

1. All configured servers are asked at the same time. The default comes from three operators that offer public time service: Cloudflare (time.cloudflare.com), the German national metrology institute PTB (ptbtime1.ptb.de, ptbtime2.ptb.de) and Microsoft (time.windows.com). The NTP Pool is one click away in the settings (0.pool.ntp.org to 3.pool.ntp.org).
2. Each answer is checked: protocol fields, kiss-of-death codes, leap indicator, stratum, root distance, freshness, a plausible date, an answer that cannot have been sent before it was received, and a random request token that a forged answer cannot know.
3. The median of the valid answers is the reference; answers too far from it are ignored as outliers. More than half of the answers must agree, otherwise nothing is changed.
4. Corrections larger than 15 minutes need at least three agreeing servers (a dead CMOS battery that reset the clock to 2001 is fixed, a single broken server claiming it is 2036 is not).
5. Differences under 2 ms are left alone. Up to 500 ms the service runs the clock 0.05 % faster or slower until the difference is gone; nobody notices. Larger differences are corrected directly.
6. If no NTP server answers at all (UDP port 123 blocked), the time is taken from the Date header of a few well-known web servers instead, accurate to about one second. This can be switched off.

Servers can be prefixed with nts:// to use Network Time Security (encrypted, authenticated NTP), for example nts://ptbtime1.ptb.de or nts://time.cloudflare.com.

The background service

ntp-sync service install (or the installer, or the button in the window):

• copies ntp-sync.exe to C:\Program Files\TimeSync,
• creates the service TimeSync: account Local Service with its own service SID, delayed automatic start, restart after a failure,
• stores settings in C:\ProgramData\TimeSync\config.json (only administrators may change them, everyone may read them) and its log and history in C:\ProgramData\TimeSync\state\,
• replaces a version 1.x service named ntp-sync, if present.

Windows Time service (w32time)

By default TimeSync takes over: the Windows Time service keeps running (other parts of Windows use it), but its time source is set to NoSync, so only one program sets the clock. The original setting is saved to C:\ProgramData\TimeSync\w32time-backup.json and written back when the service is removed. Alternatively, set w32timeMode to leave.

Company computers

On a computer that belongs to an Active Directory domain, the domain controller is the time authority and signing in depends on it. TimeSync therefore only measures and reports there, and leaves Windows Time alone. An administrator can allow it anyway (allowOnDomain, or the checkbox in the settings).

Dual boot with Linux

If the clock is off by exactly your time zone offset (one or two hours in Germany) after starting Windows, Linux and Windows disagree about the hardware clock: Linux keeps it in UTC, Windows in local time. TimeSync fixes the clock after every start, but the clean solution is to make both agree. Either tell Linux to use local time:

timedatectl set-local-rtc 1

or tell Windows to use UTC (as administrator, then restart):

reg add "HKLM\SYSTEM\CurrentControlSet\Control\TimeZoneInformation" /v RealTimeIsUniversal /t REG_DWORD /d 1 /f

To undo the Windows change:

reg delete "HKLM\SYSTEM\CurrentControlSet\Control\TimeZoneInformation" /v RealTimeIsUniversal /f

Command line

ntp-sync                      synchronize once (like "sync", with an overview)
ntp-sync check                only measure how far off the clock is (no admin needed)
ntp-sync sync                 synchronize now (via the service if it runs, else admin needed)
ntp-sync status               show the service status
ntp-sync history [-n 20]      last synchronizations and the clock's drift
ntp-sync run [-interval 60]   keep synchronizing in this window (admin)
ntp-sync diagnose [-out FILE] diagnostics report for support requests

ntp-sync service install [-interval 60]   install the background service (admin)
ntp-sync service uninstall [-purge]       remove it and restore Windows Time (admin)
ntp-sync service start|stop|restart

ntp-sync config show [-json]
ntp-sync config set servers=nts://ptbtime1.ptb.de,nts://ptbtime2.ptb.de,0.pool.ntp.org interval=30
ntp-sync config reset
ntp-sync config import FILE
ntp-sync w32time status|restore

Options: -server a,b,c (use other servers once), -json (machine readable), -force (set the clock on a domain computer), -local (don't go through the service).

Exit codes: 0 ok, 1 error, 2 wrong usage, 3 no trustworthy time, 4 administrator rights needed.

Version 1.x command lines (-install 60, -uninstall, -interval 30, -version) keep working, and a service installed by 1.x keeps running after the program file is replaced.

Settings

C:\ProgramData\TimeSync\config.json:

Key	Default	Meaning
servers	Cloudflare, PTB (2), Microsoft	NTP servers; host, host:port, nts://host
intervalMinutes	60	regular interval (15 to 1440)
syncOnStart, syncOnResume, syncOnNetworkChange	true	extra synchronizations
minCorrectionMs	2	smaller differences are left alone
slewMaxMs	500	gentle correction up to this size; 0 always steps
bigJumpMinutes	15	larger corrections need three agreeing servers
httpsFallback, httpsFallbackUrls	true, four sites	web time when NTP is blocked
w32timeMode	takeover	or leave
allowOnDomain	false	set the clock on domain computers too
eventLog	true	write to the Windows Event Viewer (source "TimeSync")

Privacy

TimeSync contacts only the time servers in your settings and, if NTP is blocked, the web servers in httpsFallbackUrls (a HEAD request, nothing is sent but the request itself). There is no telemetry, no update check and no account. The diagnostics file masks your user name.

Uninstall

Use "Apps" in the Windows settings (installer), or run ntp-sync service uninstall as administrator (portable) and delete the folder. Either way the Windows Time service is set back to how it was. -purge also deletes settings, log and history.

Building

Needs Go and, for the window, the Wails CLI v2.

git clone https://github.com/burnersen/TimeSync.git
cd TimeSync
go test ./...
for c in gui/frontend/*_check.js; do node "$c"; done
scripts/build.sh 2.0.0

Releases are built by .github/workflows/release.yml, either when a v* tag is pushed or by hand (Actions → Release → "Run workflow", enter the version). The result is a draft release with installers, zip files and SHA256SUMS.txt; it becomes public only when it is published on GitHub. Every push also leaves an unsigned test build of the x64 programs on its CI run page.

Layout: cmd/ntp-sync (command line and service), gui (the window), internal/source (asking servers), internal/engine (deciding), internal/syncer (one round), internal/service, internal/winsys (Windows calls), internal/ipc (named pipe between window and service).

License

MIT. Third-party components: THIRD_PARTY_NOTICES.md.