diff --git a/edc_glucose/model_mixins/fasting_model_mixin.py b/edc_glucose/model_mixins/fasting_model_mixin.py
index 4e02cd9..9495cee 100644
--- a/edc_glucose/model_mixins/fasting_model_mixin.py
+++ b/edc_glucose/model_mixins/fasting_model_mixin.py
@@ -3,6 +3,7 @@
 from django.db import models
 from edc_constants.choices import YES_NO_NA
 from edc_constants.constants import NOT_APPLICABLE
+from edc_model.utils import duration_hm_to_timedelta
 from edc_model.validators import hm_validator
 
 
@@ -13,6 +14,12 @@ def fasting_model_mixin_factory(
     verbose_names = verbose_names or {}
 
     class AbstractModel(models.Model):
+        def save(self, *args, **kwargs):
+            if duration_str := getattr(self, f"{prefix}fasting_duration_str", None):
+                tdelta = duration_hm_to_timedelta(duration_str)
+                setattr(self, f"{prefix}fasting_duration_delta", tdelta)
+            super().save(*args, **kwargs)
+
         class Meta:
             abstract = True
 
@@ -39,8 +46,10 @@ class Meta:
                 "For example 1h23m, 12h7m, etc"
             ),
         ),
-        f"{prefix}fasting_duration_minutes": models.IntegerField(
-            null=True, blank=True, help_text="system calculated value"
+        f"{prefix}fasting_duration_delta": models.DurationField(
+            null=True,
+            blank=True,
+            help_text="system calculated to microseconds. (hours=microseconds/3.6e+9)",
         ),
     }
 
diff --git a/edc_glucose/model_mixins/glucose_model_mixin.py b/edc_glucose/model_mixins/glucose_model_mixin.py
index 3ad9117..016dc8f 100644
--- a/edc_glucose/model_mixins/glucose_model_mixin.py
+++ b/edc_glucose/model_mixins/glucose_model_mixin.py
@@ -5,7 +5,7 @@
 from edc_lab.choices import GLUCOSE_UNITS_NA, RESULT_QUANTIFIER_NA
 
 from ..constants import GLUCOSE_HIGH_READING
-from . import fasting_model_mixin_factory
+from .fasting_model_mixin import fasting_model_mixin_factory
 
 
 def glucose_model_mixin_factory(utest_id: str, verbose_names: dict | None = None, **kwargs):
diff --git a/edc_glucose/utils.py b/edc_glucose/utils.py
index cf25540..b2e8685 100644
--- a/edc_glucose/utils.py
+++ b/edc_glucose/utils.py
@@ -7,7 +7,10 @@
 from .constants import GLUCOSE_HIGH_READING
 
 
-def validate_glucose_as_millimoles_per_liter(prefix: str, cleaned_data: dict = None) -> None:
+def validate_glucose_as_millimoles_per_liter(
+    prefix: str, cleaned_data: dict = None
+) -> None | Decimal:
+    converted_value = None
     min_val = Decimal("0.00")
     max_val = Decimal("30.00")
     high_value = Decimal(f"{GLUCOSE_HIGH_READING}")
@@ -31,3 +34,4 @@ def validate_glucose_as_millimoles_per_liter(prefix: str, cleaned_data: dict = N
                     )
                 }
             )
+    return converted_value