Record installation date and time in DB (#7334)

* Added installation date and time to the database

Information on the date and time of installation of a spec is recorded
into the database. The information is retained on reindexing.

* Expose the possibility to query for installation date

The DB can now be queried for specs that have been installed in a given
time window. This query possibility is exposed to command line via two
new options of the `find` command.

* Extended docstring for Database._add

* Use timestamps since the epoch instead of formatted date in the DB

* Allow 'pretty date' formats from command line

* Substituted kwargs with explicit arguments

* Simplified regex for pretty date strings. Added unit tests.

lib/spack/llnl/util/lang.py

@@ -29,7 +29,7 @@
 import functools
 import collections
 import inspect
-from datetime import datetime
+from datetime import datetime, timedelta
 from six import string_types
 
 # Ignore emacs backups when listing modules
@@ -442,6 +442,65 @@ def pretty_date(time, now=None):
         return str(diff) + " years ago"
 
 
+def pretty_string_to_date(date_str, now=None):
+    """Parses a string representing a date and returns a datetime object.
+
+    Args:
+        date_str (str): string representing a date. This string might be
+            in different format (like ``YYYY``, ``YYYY-MM``, ``YYYY-MM-DD``)
+            or be a *pretty date* (like ``yesterday`` or ``two months ago``)
+
+    Returns:
+        (datetime): datetime object corresponding to ``date_str``
+    """
+
+    pattern = {}
+
+    now = now or datetime.now()
+
+    # datetime formats
+    pattern[re.compile('^\d{4}$')] = lambda x: datetime.strptime(x, '%Y')
+    pattern[re.compile('^\d{4}-\d{2}$')] = lambda x: datetime.strptime(
+        x, '%Y-%m'
+    )
+    pattern[re.compile('^\d{4}-\d{2}-\d{2}$')] = lambda x: datetime.strptime(
+        x, '%Y-%m-%d'
+    )
+
+    pretty_regex = re.compile(
+        r'(a|\d+)\s*(year|month|week|day|hour|minute|second)s?\s*ago')
+
+    def _n_xxx_ago(x):
+        how_many, time_period = pretty_regex.search(x).groups()
+
+        how_many = 1 if how_many == 'a' else int(how_many)
+
+        # timedelta natively supports time periods up to 'weeks'.
+        # To apply month or year we convert to 30 and 365 days
+        if time_period == 'month':
+            how_many *= 30
+            time_period = 'day'
+        elif time_period == 'year':
+            how_many *= 365
+            time_period = 'day'
+
+        kwargs = {(time_period + 's'): how_many}
+        return now - timedelta(**kwargs)
+
+    pattern[pretty_regex] = _n_xxx_ago
+
+    # yesterday
+    callback = lambda x: now - timedelta(days=1)
+    pattern[re.compile('^yesterday$')] = callback
+
+    for regexp, parser in pattern.items():
+        if bool(regexp.match(date_str)):
+            return parser(date_str)
+
+    msg = 'date "{0}" does not match any valid format'.format(date_str)
+    raise ValueError(msg)
+
+
 class RequiredAttributeError(ValueError):
 
     def __init__(self, message):