ENH: `datetime_parse` is new transformer function.

src/caoscrawler/default_transformers.yml

-
+# Lookup table for matching functions and cfood yaml node names.
 
 submatch:
   package: caoscrawler.transformer_functions
@@ -9,3 +9,6 @@ split:
 replace:
   package: caoscrawler.transformer_functions
   function: replace
+datetime_parse:
+  package: caoscrawler.transformer_functions
+  function: datetime_parse

src/caoscrawler/transformer_functions.py

@@ -20,9 +20,14 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program. If not, see <https://www.gnu.org/licenses/>.
 
+"""Definition of default transformer functions.
+
+See https://docs.indiscale.com/caosdb-crawler/converters.html#transform-functions for more
+information.
+
 """
-Defnition of default transformer functions.
-"""
+
+import datetime
 import re
 from typing import Any
 
@@ -61,3 +66,20 @@ def replace(in_value: Any, in_parameters: dict):
     if not isinstance(in_value, str):
         raise RuntimeError("must be string")
     return in_value.replace(in_parameters['remove'], in_parameters['insert'])
+
+
+def datetime_parse(in_value: str, params: dict):
+    """Transform text so that it is formatted in a way that LinkAhead can understand it.
+
+
+Parameters
+==========
+
+- datetime_format: str, optional
+    A format string using the ``datetime`` specificaton:
+    https://docs.python.org/3/library/datetime.html#strftime-and-strptime-format-codes
+    """
+    fmt_default = "%Y-%m-%dT%H:%M:%S"
+    fmt = params.get("datetime_format", fmt_default)
+    dt_str = datetime.datetime.strptime(in_value, fmt).strftime(fmt_default)
+    return dt_str