config: new "timeadd" interpolation function

config/interpolate_funcs.go

@@ -112,6 +112,7 @@ func Funcs() map[string]ast.Function {
 		"split":        interpolationFuncSplit(),
 		"substr":       interpolationFuncSubstr(),
 		"timestamp":    interpolationFuncTimestamp(),
+		"timeadd":      interpolationFuncTimeAdd(),
 		"title":        interpolationFuncTitle(),
 		"transpose":    interpolationFuncTranspose(),
 		"trimspace":    interpolationFuncTrimSpace(),
@@ -1504,6 +1505,29 @@ func interpolationFuncTimestamp() ast.Function {
 	}
 }
 
+func interpolationFuncTimeAdd() ast.Function {
+	return ast.Function{
+		ArgTypes: []ast.Type{
+			ast.TypeString, // input timestamp string in RFC3339 format
+			ast.TypeString, // duration to add to input timestamp that should be parsable by time.ParseDuration
+		},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+
+			ts, err := time.Parse(time.RFC3339, args[0].(string))
+			if err != nil {
+				return nil, err
+			}
+			duration, err := time.ParseDuration(args[1].(string))
+			if err != nil {
+				return nil, err
+			}
+
+			return ts.Add(duration).Format(time.RFC3339), nil
+		},
+	}
+}
+
 // interpolationFuncTitle implements the "title" function that returns a copy of the
 // string in which first characters of all the words are capitalized.
 func interpolationFuncTitle() ast.Function {

config/interpolate_funcs_test.go

@@ -2426,6 +2426,38 @@ func TestInterpolateFuncTimestamp(t *testing.T) {
 	}
 }
 
+func TestInterpolateFuncTimeAdd(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${timeadd("2017-11-22T00:00:00Z", "1s")}`,
+				"2017-11-22T00:00:01Z",
+				false,
+			},
+			{
+				`${timeadd("2017-11-22T00:00:00Z", "10m1s")}`,
+				"2017-11-22T00:10:01Z",
+				false,
+			},
+			{ // also support subtraction
+				`${timeadd("2017-11-22T00:00:00Z", "-1h")}`,
+				"2017-11-21T23:00:00Z",
+				false,
+			},
+			{ // Invalid format timestamp
+				`${timeadd("2017-11-22", "-1h")}`,
+				nil,
+				true,
+			},
+			{ // Invalid format duration (day is not supported by ParseDuration)
+				`${timeadd("2017-11-22T00:00:00Z", "1d")}`,
+				nil,
+				true,
+			},
+		},
+	})
+}
+
 type testFunctionConfig struct {
 	Cases []testFunctionCase
 	Vars  map[string]ast.Variable

website/docs/configuration/interpolation.html.md

@@ -395,6 +395,9 @@ The supported built-in functions are:
    invocation of the function, so in order to prevent diffs on every plan & apply, it must be used with the
    [`ignore_changes`](/docs/configuration/resources.html#ignore-changes) lifecycle attribute.
 
+  * `timeadd(time, duration)` - Returns a UTC timestamp string corresponding to adding a given `duration` to `time` in RFC 3339 format.      
+    For example, `timeadd("2017-11-22T00:00:00Z", "10m")` produces a value `"2017-11-22T00:10:00Z"`. 
+    
   * `title(string)` - Returns a copy of the string with the first characters of all the words capitalized.
 
   * `transpose(map)` - Swaps the keys and list values in a map of lists of strings. For example, transpose(map("a", list("1", "2"), "b", list("2", "3")) produces a value equivalent to map("1", list("a"), "2", list("a", "b"), "3", list("b")).