refactor: move all datetime.now() calls to UTC

Updated all instances of datetime.now() to use timezone.utc for
consistency in handling time across different systems. This ensures that
timestamps are always in Coordinated Universal Time (UTC), avoiding
issues with time zone discrepancies and promoting uniformity in
time-related data.

Signed-off-by: Sébastien Han <seb@redhat.com>

pyproject.toml

@@ -124,14 +124,15 @@ exclude = [
 
 [tool.ruff.lint]
 select = [
-    "B",  # flake8-bugbear
-    "B9", # flake8-bugbear subset
-    "C",  # comprehensions
-    "E",  # pycodestyle
-    "F",  # Pyflakes
-    "N",  # Naming
-    "W",  # Warnings
-    "I",  # isort
+    "B",   # flake8-bugbear
+    "B9",  # flake8-bugbear subset
+    "C",   # comprehensions
+    "E",   # pycodestyle
+    "F",   # Pyflakes
+    "N",   # Naming
+    "W",   # Warnings
+    "I",   # isort
+    "DTZ", # datetime rules
 ]
 ignore = [
     # The following ignores are desired by the project maintainers.
@@ -145,6 +146,10 @@ ignore = [
     "C901", # Complexity of the function is too high
 ]
 
+# Ignore the following errors for the following files
+[tool.ruff.lint.per-file-ignores]
+"tests/**/*.py" = ["DTZ"] # Ignore datetime rules for tests
+
 [tool.mypy]
 mypy_path = ["llama_stack"]
 packages = ["llama_stack"]