diff --git a/go/buildstep/buildhours.go b/go/buildstep/buildhours.go
new file mode 100644
index 0000000..e0bc991
--- /dev/null
+++ b/go/buildstep/buildhours.go
@@ -0,0 +1,135 @@
+package buildstep
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+)
+
+// **A node may build only within its build hours** (the operator, 2026-10-26 21:18: "we should only use g14 as a builder
+// outside office hours", "from 18 o'clock until 6 in the morning"). A build module's `build-hours` setting names, per
+// node, the window in the node's local time in which it takes asks; outside it the capacity judged by is 0: steps running
+// finish, no new one is taken. A node it does not name, or an empty setting, builds at any hour. The puller judges it at
+// every judgement and wakes on one timer to the window's next edge (a clock edge, not a poll of another part).
+
+// BuildHoursSetting is the build modules' setting of the nodes' build windows: `<node>=HH:MM-HH:MM` pairs separated by
+// commas, in the node's local time; an end before its start crosses midnight (g14=18:00-06:00).
+const BuildHoursSetting = "build-hours"
+
+// BuildWindow is one node's build hours: minutes after local midnight, Start inclusive, End exclusive.
+type BuildWindow struct {
+	Start, End int
+}
+
+// String is the window as the setting writes it.
+func (w BuildWindow) String() string {
+	return fmt.Sprintf("%02d:%02d-%02d:%02d", w.Start/60, w.Start%60, w.End/60, w.End%60)
+}
+
+// Open says whether t, in its own location, falls in the window.
+func (w BuildWindow) Open(t time.Time) bool {
+	m := t.Hour()*60 + t.Minute()
+	if w.Start < w.End {
+		return m >= w.Start && m < w.End
+	}
+	return m >= w.Start || m < w.End
+}
+
+// NextEdge is the first time after t, in t's location, at which the window opens or closes.
+func (w BuildWindow) NextEdge(t time.Time) time.Time {
+	var next time.Time
+	for day := 0; day <= 1; day++ {
+		for _, m := range []int{w.Start, w.End} {
+			e := time.Date(t.Year(), t.Month(), t.Day()+day, m/60, m%60, 0, 0, t.Location())
+			if e.After(t) && (next.IsZero() || e.Before(next)) {
+				next = e
+			}
+		}
+	}
+	return next
+}
+
+// ParseBuildHours is node's window in a build-hours setting: nil when the setting is empty or does not name the node.
+func ParseBuildHours(setting, node string) (*BuildWindow, error) {
+	var found *BuildWindow
+	for _, pair := range strings.Split(setting, ",") {
+		pair = strings.TrimSpace(pair)
+		if pair == "" {
+			continue
+		}
+		name, span, ok := strings.Cut(pair, "=")
+		from, to, ok2 := strings.Cut(span, "-")
+		if !ok || !ok2 || strings.TrimSpace(name) == "" {
+			return nil, fmt.Errorf("the build hours %q: %q is not <node>=HH:MM-HH:MM", setting, pair)
+		}
+		start, err := minuteOf(from)
+		if err != nil {
+			return nil, fmt.Errorf("the build hours %q: %v", setting, err)
+		}
+		end, err := minuteOf(to)
+		if err != nil {
+			return nil, fmt.Errorf("the build hours %q: %v", setting, err)
+		}
+		if start == end {
+			return nil, fmt.Errorf("the build hours %q: %s opens and closes at the same minute", setting, pair)
+		}
+		if strings.TrimSpace(name) == node {
+			found = &BuildWindow{Start: start, End: end}
+		}
+	}
+	return found, nil
+}
+
+// minuteOf reads HH:MM as minutes after midnight.
+func minuteOf(s string) (int, error) {
+	var h, m int
+	s = strings.TrimSpace(s)
+	if n, err := fmt.Sscanf(s, "%d:%d", &h, &m); err != nil || n != 2 || h < 0 || h > 23 || m < 0 || m > 59 || len(s) != 5 {
+		return 0, fmt.Errorf("%q is not a time HH:MM", s)
+	}
+	return h*60 + m, nil
+}
+
+// NodeBuildHours is node's build window, read from the module's settings file at path at every call, and why it is
+// none when the value cannot be read. No path, no file value or an empty one is no window and no fault. A value that
+// cannot be read is no window too, so the node builds at any hour; the setting is a preference of hours, not a safety
+// bound (a node that must never build has capacity 0). It is loud: the fault is said once per distinct fault through
+// logf, and the puller writes it in the capacity record that build_usage shows.
+func NodeBuildHours(path, node string, logf func(string, ...any)) func() (*BuildWindow, string) {
+	said := ""
+	say := func(why string) {
+		if logf != nil && why != said {
+			said = why
+			logf("%s, so %s builds at any hour", why, node)
+		}
+	}
+	fault := func(why string) (*BuildWindow, string) {
+		say(why)
+		return nil, why
+	}
+	return func() (*BuildWindow, string) {
+		if path == "" {
+			return nil, ""
+		}
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			return fault(fmt.Sprintf("the setting %s cannot be read: %v", BuildHoursSetting, err))
+		}
+		var s map[string]json.RawMessage
+		if err := json.Unmarshal(raw, &s); err != nil {
+			return fault(fmt.Sprintf("the settings file %s is not JSON: %v", path, err))
+		}
+		setting := ""
+		if v, ok := s[BuildHoursSetting]; ok && json.Unmarshal(v, &setting) != nil {
+			return fault(fmt.Sprintf("the setting %s %s is not a string", BuildHoursSetting, v))
+		}
+		w, err := ParseBuildHours(setting, node)
+		if err != nil {
+			return fault(err.Error())
+		}
+		said = ""
+		return w, ""
+	}
+}
diff --git a/go/buildstep/capacity_test.go b/go/buildstep/capacity_test.go
index c52f15e..c44bbb5 100644
--- a/go/buildstep/capacity_test.go
+++ b/go/buildstep/capacity_test.go
@@ -3,7 +3,9 @@ package buildstep
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
+	"time"
 )
 
 // 1887/A7: the node's build capacity is the build module's setting, read at every pull: the control-node at 0 pulls
@@ -84,4 +86,52 @@ func TestNodeLimitsAndTheCPUBound(t *testing.T) {
 	if len(said) != 1 {
 		t.Errorf("a value not above zero is said once, said %d times", len(said))
 	}
+
+	// The operator, 2026-10-26 21:18: g14 builds only from 18:00 to 06:00, its local time. Outside its build hours the
+	// capacity judged by is 0, said with the window and when it opens; a node not named, or a bad value, builds at any hour.
+	cet := time.FixedZone("CET", 3600)
+	day := func(hh, mm int) time.Time { return time.Date(2026, 10, 26, hh, mm, 0, 0, cet) }
+	was := now
+	defer func() { now = was }()
+	for _, c := range []struct {
+		name, setting, node string
+		at                  time.Time
+		effective           int
+		edge                string // the window's next edge, HH:MM; "" for no window
+		bad                 bool
+	}{
+		{"g14 in the office", "g14=18:00-06:00", "g14", day(10, 0), 0, "18:00", false},
+		{"g14 in the evening", "g14=18:00-06:00", "g14", day(18, 0), 2, "06:00", false},
+		{"g14 past midnight", "g14=18:00-06:00", "g14", day(5, 59), 2, "06:00", false},
+		{"g14 at the close", "g14=18:00-06:00", "g14", day(6, 0), 0, "18:00", false},
+		{"a day window", "shanks=09:00-17:30,g14=18:00-06:00", "shanks", day(17, 29), 2, "17:30", false},
+		{"a node not named", "g14=18:00-06:00", "ace", day(10, 0), 2, "", false},
+		{"a bad time", "g14=18:00-6", "g14", day(10, 0), 2, "", true},
+		{"no span", "g14=18:00-18:00", "g14", day(10, 0), 2, "", true},
+	} {
+		w, err := ParseBuildHours(c.setting, c.node)
+		if (err != nil) != c.bad {
+			t.Errorf("%s: parse %v", c.name, err)
+		}
+		if edge := ""; w != nil {
+			edge = w.NextEdge(c.at).Format("15:04")
+			if edge != c.edge {
+				t.Errorf("%s: next edge %s, want %s", c.name, edge, c.edge)
+			}
+		} else if c.edge != "" {
+			t.Errorf("%s: no window, want one", c.name)
+		}
+		file := filepath.Join(t.TempDir(), "settings.json")
+		os.WriteFile(file, []byte(`{"build-hours":"`+c.setting+`"}`), 0o600)
+		now = func() time.Time { return c.at }
+		p := &Puller{Language: "go", Node: c.node, Capacity: func() int { return 2 }, BuildHours: NodeBuildHours(file, c.node, nil)}
+		got := p.CapacityDetail()
+		if got.Effective != c.effective || got.OutsideHours != (c.effective == 0) || (c.effective == 0 && !strings.Contains(got.Why, "outside its build hours 18:00-06:00 CET until 18:00")) {
+			t.Errorf("%s: %+v, want capacity %d", c.name, got, c.effective)
+		}
+		// A value that cannot be read builds at any hour, loudly: the record names the value and says why.
+		if c.bad && (!strings.Contains(got.BuildHoursFault, c.setting) || !strings.Contains(got.Why, "at any hour, because its build hours cannot be read")) {
+			t.Errorf("%s: a bad value is not said in the record: %+v", c.name, got)
+		}
+	}
 }
diff --git a/go/buildstep/pull.go b/go/buildstep/pull.go
index 87b95ee..893d631 100644
--- a/go/buildstep/pull.go
+++ b/go/buildstep/pull.go
@@ -82,6 +82,10 @@ type Puller struct {
 	// (EffectiveCapacity). Nil bounds nothing by the CPU. Quota reads the quota (Quota when nil; a test's own).
 	CoresPerStep func() float64
 	Quota        func() (percent int, from string)
+	// BuildHours is the node's build window, read at every judgement (NodeBuildHours); with WatchConfig set the value
+	// mesh-config resolves for `build-hours` takes its place once handed. Outside the window the capacity judged by is 0:
+	// steps running finish, no new one is taken; the puller wakes at the window's next edge. Nil builds at any hour.
+	BuildHours func() (*BuildWindow, string)
 	// Has says whether this node has a need an ask names (delivery.AskNeeds, issue 890). An ask needing what it lacks is
 	// declined: its step ends refused as declined (delivery.StepDeclined), the message is dropped for its asker to put
 	// again, and the worker it came from is not looked at again until the next pull's wait, so a node lacking the need
@@ -97,6 +101,11 @@ type Puller struct {
 	// watched is the capacity the config watch handed last, used in place of Capacity once hasWatched.
 	watched    int
 	hasWatched bool
+	// hours is the build window the config watch handed last, and hoursFault why its value cannot be read; used in place
+	// of BuildHours once hasHours.
+	hours      *BuildWindow
+	hoursFault string
+	hasHours   bool
 	// judged is the capacity said last (log and capacity.<node>), so a change is said once.
 	judged     delivery.Capacity
 	hasJudged  bool
@@ -265,16 +274,59 @@ func (p *Puller) CapacityDetail() delivery.Capacity {
 		c = EffectiveCapacity(configured, percent, qfrom, p.CoresPerStep())
 	}
 	c.ConfiguredFrom = from
+	w, fault := p.window()
+	if fault != "" {
+		c.BuildHoursFault = fault
+		c.Why = fmt.Sprintf("%s; at any hour, because its build hours cannot be read: %s", c.Why, fault)
+	}
+	if w != nil {
+		at := now()
+		zone, _ := at.Zone()
+		c.BuildHours = w.String() + " " + zone
+		if !w.Open(at) && c.Effective > 0 {
+			c.OutsideHours = true
+			c.Why = fmt.Sprintf("0, outside its build hours %s until %s (otherwise %s)", c.BuildHours, w.NextEdge(at).Format("15:04"), c.Why)
+			c.Effective = 0
+		} else if c.Effective > 0 {
+			c.Why = fmt.Sprintf("%s; within its build hours %s until %s", c.Why, c.BuildHours, w.NextEdge(at).Format("15:04"))
+		}
+	}
 	p.sayCapacity(c)
 	return c
 }
 
+// window is the node's build window now (the config watch's last value, else BuildHours, else none), and why its value
+// cannot be read when it cannot.
+func (p *Puller) window() (*BuildWindow, string) {
+	p.mu.Lock()
+	w, fault, has := p.hours, p.hoursFault, p.hasHours
+	p.mu.Unlock()
+	if has {
+		return w, fault
+	}
+	if p.BuildHours != nil {
+		return p.BuildHours()
+	}
+	return nil, ""
+}
+
+// untilEdge is how long until the build window's next edge, and false when the node has no window.
+func (p *Puller) untilEdge() (time.Duration, bool) {
+	w, _ := p.window()
+	if w == nil {
+		return 0, false
+	}
+	at := now()
+	return w.NextEdge(at).Sub(at), true
+}
+
 // sayCapacity logs c when it differs from what was said last, and writes it to the node's capacity key until a write
 // lands (a failed write is tried again at the next judgement, its error said once).
 func (p *Puller) sayCapacity(c delivery.Capacity) {
 	p.mu.Lock()
 	same := p.hasJudged && p.judged.Effective == c.Effective && p.judged.Configured == c.Configured && p.judged.ByCPU == c.ByCPU &&
-		p.judged.CoresPerStep == c.CoresPerStep && p.judged.QuotaCores == c.QuotaCores
+		p.judged.CoresPerStep == c.CoresPerStep && p.judged.QuotaCores == c.QuotaCores && p.judged.OutsideHours == c.OutsideHours &&
+		p.judged.BuildHours == c.BuildHours && p.judged.BuildHoursFault == c.BuildHoursFault
 	if !same {
 		p.judged, p.hasJudged, p.capWritten = c, true, false
 	}
@@ -320,6 +372,7 @@ func (p *Puller) Pull(ctx context.Context) error {
 		p.logf("the pause of %s cannot be read: %v; pulling as not paused", p.Node, err)
 	}
 	go p.watchCapacity() // WatchConfigServed waits for Serve, which a module starts after its pull
+	go p.watchHours()
 	wait := p.PullWait
 	if wait <= 0 {
 		wait = DefaultPullWait
@@ -332,16 +385,29 @@ func (p *Puller) Pull(ctx context.Context) error {
 			// Parked until something wakes it: a capacity change (the config watch), a resume, or a step's end. Every
 			// park logs once, so a puller that sits idle says why (issue 1887).
 			p.logf("the %s puller on %s parks: %d of capacity %d in hand, paused %v", p.Language, p.Node, inHand, capacity, p.Paused())
+			var edge <-chan time.Time // the build window's next edge, when the node has one: one timer, not a poll
+			stop := func() bool { return false }
+			if d, ok := p.untilEdge(); ok {
+				t := time.NewTimer(d + time.Second)
+				edge, stop = t.C, t.Stop
+			}
 			select {
 			case <-ctx.Done():
+				stop()
 				return ctx.Err()
 			case <-p.wake:
+			case <-edge:
 			}
+			stop()
 			_, capacity, inHand = p.roomNow()
 			p.logf("the %s puller on %s wakes: %d of capacity %d in hand, paused %v", p.Language, p.Node, inHand, capacity, p.Paused())
 			continue
 		}
-		msg, err := p.next(ctx, wait)
+		pullWait := wait
+		if d, ok := p.untilEdge(); ok && d < pullWait {
+			pullWait = d // a pull does not wait past the window's close
+		}
+		msg, err := p.next(ctx, pullWait)
 		if err != nil {
 			if ctx.Err() != nil {
 				return ctx.Err()
@@ -560,6 +626,42 @@ func (p *Puller) watchCapacity() {
 	}
 }
 
+// watchHours follows the module's config value `build-hours` when WatchConfig and BuildHours are set, as watchCapacity
+// follows `capacity`: each value handed is kept, used in place of BuildHours, and wakes the puller. A value that cannot
+// be read is no window, said; a watch that cannot start leaves the puller on BuildHours, said once.
+func (p *Puller) watchHours() {
+	if p.WatchConfig == nil || p.BuildHours == nil {
+		return
+	}
+	err := p.WatchConfig(BuildHoursSetting, func(c stdio.ProvisionChange) error {
+		setting := ""
+		var w *BuildWindow
+		var why string
+		switch {
+		case c.Refused != "":
+			why = "its build hours are no longer answered: " + c.Refused
+		case len(c.Value) > 0 && string(c.Value) != "null" && json.Unmarshal(c.Value, &setting) != nil:
+			why = fmt.Sprintf("the setting %s %s is not a string", BuildHoursSetting, c.Value)
+		default:
+			var err error
+			if w, err = ParseBuildHours(setting, p.Node); err != nil {
+				why = err.Error()
+			}
+		}
+		p.mu.Lock()
+		p.hours, p.hoursFault, p.hasHours = w, why, true
+		p.mu.Unlock()
+		if why != "" {
+			p.logf("the %s puller on %s builds at any hour: %s", p.Language, p.Node, why)
+		}
+		p.Wake()
+		return nil
+	})
+	if err != nil {
+		p.logf("the %s build hours of %s cannot be followed on the bus, so they are read from the settings file: %v", p.Language, p.Node, err)
+	}
+}
+
 // WatchConfigServed is stdio.WatchConfig once Serve has the runtime's channel (stdio.Served): a build module starts its
 // pull before it serves, and a watch asked before that would be refused.
 func WatchConfigServed(key string, handle func(stdio.ProvisionChange) error) error {
diff --git a/go/delivery/stepasks.go b/go/delivery/stepasks.go
index 33ac861..6b8956a 100644
--- a/go/delivery/stepasks.go
+++ b/go/delivery/stepasks.go
@@ -724,6 +724,12 @@ type Capacity struct {
 	// when no quota is known.
 	CoresPerStep float64 `json:"cores-per-step"`
 	ByCPU        int     `json:"by-cpu,omitempty"`
+	// BuildHours is the node's build window (HH:MM-HH:MM, its local time and zone), absent when it builds at any hour;
+	// OutsideHours says the time judged fell outside it, so Effective is 0 and the next step waits for the window to open.
+	BuildHours   string `json:"build-hours,omitempty"`
+	OutsideHours bool   `json:"outside-hours,omitempty"`
+	// BuildHoursFault is why the node's build-hours value cannot be read, naming the value: it then builds at any hour.
+	BuildHoursFault string `json:"build-hours-fault,omitempty"`
 	// Why is the capacity in words: what bounds it.
 	Why string    `json:"why"`
 	At  time.Time `json:"at,omitzero"`