Fragile datetime parsing via string slicing

## Problem
`data_diff/databases/base.py:1000` — Datetime parsing uses hardcoded string slicing instead of proper parsing.

## Impact
Fragile and may break with unexpected datetime formats from different databases.

## Suggested Fix
Use `datetime.strptime()` or `dateutil.parser` for robust datetime parsing.

## References
- File: `data_diff/databases/base.py`, around line 1000