part #2

src/AirGap/StellaOps.AirGap.Time/Services/TimeAnchorPolicyService.Drift.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StellaOps.AirGap.Time.Services;
+
+public sealed partial class TimeAnchorPolicyService
+{
+    public async Task<TimeAnchorDriftResult> CalculateDriftAsync(
+        string tenantId,
+        DateTimeOffset targetTime,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(tenantId);
+
+        var now = _timeProvider.GetUtcNow();
+        var status = await _statusService.GetStatusAsync(tenantId, now, cancellationToken).ConfigureAwait(false);
+
+        if (!status.HasAnchor)
+        {
+            return new TimeAnchorDriftResult(
+                HasAnchor: false,
+                Drift: TimeSpan.Zero,
+                DriftExceedsThreshold: false,
+                AnchorTime: null);
+        }
+
+        var drift = targetTime - status.Anchor!.AnchorTime;
+        var absDriftSeconds = Math.Abs(drift.TotalSeconds);
+        var exceedsThreshold = absDriftSeconds > _options.MaxDriftSeconds;
+
+        return new TimeAnchorDriftResult(
+            HasAnchor: true,
+            Drift: drift,
+            DriftExceedsThreshold: exceedsThreshold,
+            AnchorTime: status.Anchor.AnchorTime);
+    }
+}