diff --git a/src/libs/zbxalgo/prediction.c b/src/libs/zbxalgo/prediction.c
index 438d7bc8200..26b560e1d1f 100644
--- a/src/libs/zbxalgo/prediction.c
+++ b/src/libs/zbxalgo/prediction.c
@@ -424,7 +424,7 @@ static int	zbx_regression(double *t, double *x, int n, zbx_fit_t fit, int k, zbx
 	{
 		if (0 == isfinite(coefficients->elements[i]))
 		{
-			zabbix_log(LOG_LEVEL_DEBUG, "regression produced non-finite coefficient");
+			zabbix_log(LOG_LEVEL_DEBUG, "%s(): regression produced non-finite coefficient", __func__);
 			res = FAIL;
 			goto out;
 		}
@@ -979,7 +979,7 @@ static void	zbx_log_expression(double now, zbx_fit_t fit, int k, zbx_matrix_t *c
 double	zbx_forecast(double *t, double *x, int n, double now, double time, zbx_fit_t fit, unsigned k, zbx_mode_t mode)
 {
 	zbx_matrix_t	*coefficients = NULL;
-	double		left, right, result;
+	double		result;
 	int		res;
 
 	if (1 == n)
@@ -1007,20 +1007,32 @@ double	zbx_forecast(double *t, double *x, int n, double now, double time, zbx_fi
 		/* linear 2-point estimate for linear and logarithmic fits in value mode.     */
 		if ((FIT_LINEAR == fit || FIT_LOGARITHMIC == fit) && MODE_VALUE == mode)
 		{
-			int	i_min_t = 0, i_max_t = 0;
-			double	t_span;
+			int	i_min_t, i_max_t;
+			double	delta_t;
 
-			for (int i = 1; i < n; i++)
+			/* The value cache may return data in either ascending or descending */
+			/* time order. Find indices of min and max timestamp.                */
+
+			if (t[0] < t[n-1])
+			{
+				i_min_t = 0;
+				i_max_t = n - 1;
+			}
+			else if (t[0] > t[n-1])
 			{
-				if (t[i] < t[i_min_t])	i_min_t = i;
-				if (t[i] > t[i_max_t])	i_max_t = i;
+				i_min_t = n - 1;
+				i_max_t = 0;
 			}
+			else
+				i_min_t = i_max_t = 0;
 
-			if (0.0 < (t_span = t[i_max_t] - t[i_min_t]))
+			if (0.0 < (delta_t = t[i_max_t] - t[i_min_t]))
 			{
-				double	slope = (x[i_max_t] - x[i_min_t]) / t_span;
+				double	slope = (x[i_max_t] - x[i_min_t]) / delta_t;
 
 				result = x[i_max_t] + slope * (now + time - t[i_max_t]);
+				
+				/* NaN and +/-Inf in 'result' are handled below, on return */
 				res = SUCCEED;
 			}
 		}
@@ -1058,6 +1070,8 @@ double	zbx_forecast(double *t, double *x, int n, double now, double time, zbx_fi
 
 	if (FIT_LINEAR == fit || FIT_EXPONENTIAL == fit || FIT_LOGARITHMIC == fit || FIT_POWER == fit)
 	{
+		double	left, right;
+
 		/* fit is monotone, therefore maximum and minimum are either at now or at now + time */
 		if (SUCCEED != zbx_calculate_value(now, coefficients, fit, &left) ||
 				SUCCEED != zbx_calculate_value(now + time, coefficients, fit, &right))
@@ -1178,42 +1192,75 @@ double	zbx_timeleft(double *t, double *x, int n, double now, double threshold, z
 
 	if (SUCCEED != (res = zbx_regression(t, x, n, fit, k, coefficients)))
 	{
-		/* When regression fails due to overflow with extreme values, fall back to a linear        */
-		/* 2-point slope estimate using the oldest and newest data points. The value cache may    */
-		/* return data in either ascending or descending time order, so scan for the actual       */
-		/* min-t and max-t indices rather than assuming array ordering.                           */
+		/* When regression fails due to overflow with extreme values, fall back to a linear */
+		/* 2-point slope estimate using the oldest and newest data points.                  */
+
 		if (FIT_LINEAR == fit || FIT_LOGARITHMIC == fit)
 		{
-			int	i_min_t = 0, i_max_t = 0;
-			double	t_span, slope, val_now;
+			int	i_min_t, i_max_t;
+			double	delta_t;
 
-			for (int i = 1; i < n; i++)
+			/* The value cache may return data in either ascending or descending */
+			/* time order. Find indices of min and max timestamp.                */
+
+			if (t[0] < t[n-1])
+			{
+				i_min_t = 0;
+				i_max_t = n - 1;
+			}
+			else if (t[0] > t[n-1])
 			{
-				if (t[i] < t[i_min_t])	i_min_t = i;
-				if (t[i] > t[i_max_t])	i_max_t = i;
+				i_min_t = n - 1;
+				i_max_t = 0;
 			}
+			else
+				i_min_t = i_max_t = 0;
 
-			if (0.0 < (t_span = t[i_max_t] - t[i_min_t]))
+			if (0.0 < (delta_t = t[i_max_t] - t[i_min_t]))
 			{
-				slope = (x[i_max_t] - x[i_min_t]) / t_span;
+				double	slope = (x[i_max_t] - x[i_min_t]) / delta_t;
 
 				if (0 == isfinite(slope))
 				{
 					/* Slope overflowed — use trend direction and newest value to decide. */
 					result = (x[i_max_t] > x[i_min_t]) ?
 							((x[i_max_t] >= threshold) ? DBL_MAX : 0.0) :
-							((x[i_max_t] <= threshold) ? 0.0 : DBL_MAX);
+							((x[i_max_t] <= threshold) ? DBL_MAX : 0.0);
 				}
 				else
 				{
-					val_now = x[i_max_t] + slope * (now - t[i_max_t]);
-
-					if (0 != isfinite(val_now) && val_now > threshold && 0.0 > slope)
-						result = (threshold - val_now) / slope;
-					else if (val_now > threshold)
-						result = DBL_MAX;
+					double	val_now = x[i_max_t] + slope * (now - t[i_max_t]);
+
+					if (0 != isnan(val_now))
+						 goto out;
+
+					if (0 != isfinite(val_now))
+					{
+						if (val_now > threshold)
+						{
+							if (0.0 > slope)
+								result = (threshold - val_now) / slope;
+							else
+								result = DBL_MAX;
+						}
+						else if (val_now < threshold)
+						{
+							if (0.0 < slope)
+								result = (threshold - val_now) / slope;
+							else
+								result = DBL_MAX;
+						}
+						else
+							result = 0.0;
+					}
 					else
-						result = 0.0;
+					{
+						/* val_now overflowed — use trend direction and newest */
+						/* value to decide. */
+						result = (x[i_max_t] > x[i_min_t]) ?
+								((x[i_max_t] >= threshold) ? DBL_MAX : 0.0) :
+								((x[i_max_t] <= threshold) ? DBL_MAX : 0.0);
+					}
 				}
 
 				res = SUCCEED;
diff --git a/src/libs/zbxcalc/func_eval.c b/src/libs/zbxcalc/func_eval.c
index c1fba0fc802..0798b1be15c 100644
--- a/src/libs/zbxcalc/func_eval.c
+++ b/src/libs/zbxcalc/func_eval.c
@@ -2294,13 +2294,10 @@ clean:
 static int	evaluate_FORECAST(zbx_variant_t *value, const zbx_dc_evaluate_item_t *item, const char *parameters,
 		const zbx_timespec_t *ts, zbx_history_selector_t *selector, char **error)
 {
-	char				*fit_str = NULL, *mode_str = NULL;
-	double				*t = NULL, *x = NULL;
 	int				nparams, i, ret = FAIL, seconds = 0, nvalues = 0;
 	zbx_history_selector_t		time_selector = {0};
 	unsigned int			k = 0;
 	zbx_vector_history_record_t	values;
-	zbx_timespec_t			zero_time;
 	zbx_fit_t			fit;
 	zbx_mode_t			mode;
 	zbx_timespec_t			ts_end = *ts;
@@ -2343,12 +2340,17 @@ static int	evaluate_FORECAST(zbx_variant_t *value, const zbx_dc_evaluate_item_t
 
 	if (3 <= nparams)
 	{
+		char	*fit_str = NULL;
+
 		if (SUCCEED != get_function_parameter_str(parameters, 3, &fit_str) ||
 				SUCCEED != zbx_fit_code(fit_str, &fit, &k, error))
 		{
 			*error = zbx_strdup(*error, "invalid fourth parameter");
+			zbx_free(fit_str);
 			goto out;
 		}
+
+		zbx_free(fit_str);
 	}
 	else
 	{
@@ -2357,12 +2359,17 @@ static int	evaluate_FORECAST(zbx_variant_t *value, const zbx_dc_evaluate_item_t
 
 	if (4 == nparams)
 	{
+		char	*mode_str = NULL;
+
 		if (SUCCEED != get_function_parameter_str(parameters, 4, &mode_str) ||
 				SUCCEED != zbx_mode_code(mode_str, &mode, error))
 		{
 			*error = zbx_strdup(*error, "invalid fifth parameter");
+			zbx_free(mode_str);
 			goto out;
 		}
+
+		zbx_free(mode_str);
 	}
 	else
 	{
@@ -2391,8 +2398,10 @@ static int	evaluate_FORECAST(zbx_variant_t *value, const zbx_dc_evaluate_item_t
 
 	if (0 < values.values_num)
 	{
-		t = (double *)zbx_malloc(t, (size_t)values.values_num * sizeof(double));
-		x = (double *)zbx_malloc(x, (size_t)values.values_num * sizeof(double));
+		zbx_timespec_t	zero_time;
+
+		double	*t = (double *)zbx_malloc(t, (size_t)values.values_num * sizeof(double));
+		double	*x = (double *)zbx_malloc(x, (size_t)values.values_num * sizeof(double));
 
 		zero_time.sec = values.values[values.values_num - 1].timestamp.sec;
 		zero_time.ns = values.values[values.values_num - 1].timestamp.ns;
@@ -2419,6 +2428,8 @@ static int	evaluate_FORECAST(zbx_variant_t *value, const zbx_dc_evaluate_item_t
 		zbx_variant_set_dbl(value, zbx_forecast(t, x, values.values_num,
 				ts->sec - zero_time.sec - 1.0e-9 * (zero_time.ns + 1), time_selector.value, fit, k,
 				mode));
+		zbx_free(x);
+		zbx_free(t);
 	}
 	else
 	{
@@ -2429,12 +2440,6 @@ static int	evaluate_FORECAST(zbx_variant_t *value, const zbx_dc_evaluate_item_t
 out:
 	zbx_history_record_vector_destroy(&values, item->value_type);
 
-	zbx_free(fit_str);
-	zbx_free(mode_str);
-
-	zbx_free(t);
-	zbx_free(x);
-
 	zabbix_log(LOG_LEVEL_DEBUG, "End of %s():%s", __func__, zbx_result_string(ret));
 
 	return ret;
diff --git a/tests/libs/zbxcalc/zbx_evaluate_function.yaml b/tests/libs/zbxcalc/zbx_evaluate_function.yaml
index 5000bcee522..838a79caff1 100644
--- a/tests/libs/zbxcalc/zbx_evaluate_function.yaml
+++ b/tests/libs/zbxcalc/zbx_evaluate_function.yaml
@@ -1637,7 +1637,7 @@ out:
   return: SUCCEED
   value: 1.7976931348623158e+308
 ---
-test case: Evaluate timeleft(#2,wait 0)
+test case: Evaluate timeleft(#2,-1) from DBL_MAX to 0, threshold -1 in the past
 in:
   history:
   - itemid: 1
@@ -1650,10 +1650,44 @@ in:
   time: 2017-01-10 10:03:00.000000000 +00:00
   function: timeleft
   params: '#2,-1'
+out:
+  return: SUCCEED
+  value: 1.7976931348623158e+308
+---
+test case: Evaluate timeleft(#2,-1) from DBL_MAX to 0, threshold -1 practically instantly reached
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 1.7976931348623158e+308
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 0.0
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+  time: 2017-01-10 10:02:00.000000001 +00:00
+  function: timeleft
+  params: '#2,-1'
 out:
   return: SUCCEED
   value: 0
 ---
+test case: Evaluate timeleft(#2,-1) from 0 to DBL_MAX, threshold -1 never
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 0.0
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 1.7976931348623158e+308
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+  time: 2017-01-10 10:03:00.000000000 +00:00
+  function: timeleft
+  params: '#2,-1'
+out:
+  return: SUCCEED
+  value: 1.7976931348623158e+308
+---
 test case: Evaluate timeleft(5m,65)
 in:
   history:
@@ -4826,6 +4860,26 @@ in:
 out:
   return: SUCCEED
   value: 272384191.768584
+---
+test case: Evaluate timeleft(#3,1.7976931348623158e+308,"logarithmic") returning 0
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 1.7976931348623155e+308
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 1.7976931348623156e+308
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+    - value: 1.7976931348623157e+308
+      ts: 2017-01-10 10:03:00.000000000 +00:00
+  time: 2017-01-10 10:04:00.000000000 +00:00
+  function: timeleft
+  params: '#3,1.7976931348623158e+308,"logarithmic"'
+out:
+  return: SUCCEED
+  value: 0
+---
 test case: Evaluate timeleft(#3,1.7976931348623158e+308,"logarithmic")
 in:
   history:
@@ -5702,4 +5756,96 @@ db data:
   - [10.0, 9]
   trends (10):
   - [2.2250738585072014e-308, 10]
+---
+test case: Evaluate timeleft(#5,6.0, 60)
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 1.0
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 2.0
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+    - value: 3.0
+      ts: 2017-01-10 10:03:00.000000000 +00:00
+    - value: 4.0
+      ts: 2017-01-10 10:04:00.000000000 +00:00
+    - value: 5.0
+      ts: 2017-01-10 10:05:00.000000000 +00:00
+  time: 2017-01-10 10:05:00.000000000 +00:00
+  function: timeleft
+  params: '#5,6.0'
+out:
+  return: SUCCEED
+  value: 60
+---
+test case: Evaluate timeleft(#5,6.0, 1)
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 1.0
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 2.0
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+    - value: 3.0
+      ts: 2017-01-10 10:03:00.000000000 +00:00
+    - value: 4.0
+      ts: 2017-01-10 10:04:00.000000000 +00:00
+    - value: 5.0
+      ts: 2017-01-10 10:05:00.000000000 +00:00
+  time: 2017-01-10 10:05:59.000000000 +00:00
+  function: timeleft
+  params: '#5,6.0'
+out:
+  return: SUCCEED
+  value: 1
+---
+test case: Evaluate timeleft(#5,6.0, 0)
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 1.0
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 2.0
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+    - value: 3.0
+      ts: 2017-01-10 10:03:00.000000000 +00:00
+    - value: 4.0
+      ts: 2017-01-10 10:04:00.000000000 +00:00
+    - value: 5.0
+      ts: 2017-01-10 10:05:00.000000000 +00:00
+  time: 2017-01-10 10:06:00.000000000 +00:00
+  function: timeleft
+  params: '#5,6.0'
+out:
+  return: SUCCEED
+  value: 0
+---
+test case: Evaluate timeleft(#5,6.0, 1.7976931348623158e+308)
+in:
+  history:
+  - itemid: 1
+    value type: ITEM_VALUE_TYPE_FLOAT
+    data:
+    - value: 1.0
+      ts: 2017-01-10 10:01:00.000000000 +00:00
+    - value: 2.0
+      ts: 2017-01-10 10:02:00.000000000 +00:00
+    - value: 3.0
+      ts: 2017-01-10 10:03:00.000000000 +00:00
+    - value: 4.0
+      ts: 2017-01-10 10:04:00.000000000 +00:00
+    - value: 5.0
+      ts: 2017-01-10 10:05:00.000000000 +00:00
+  time: 2017-01-10 10:06:01.000000000 +00:00
+  function: timeleft
+  params: '#5,6.0'
+out:
+  return: SUCCEED
+  value: 1.7976931348623158e+308
 ...