PostgreSQL uses a format similar to IS8601 but allows values expressed
with tenths or hundreths of seconds rather than just milliseconds.
`2020-01-01 12:34:98.3+00` as opposed to `2020-01-01 12:34:98.300+00`,
for example. The `times` module in the Nim stdlib supports only
milliseconds with exactly three digits. To bridge this gap we detect the
two unsupported cases and pad the fractional seconds out to millisecond
precision.
