Fix resolve_time to be Pandas 3 compatible

[seanshahkarami]

I received a ticket from Greg Anderson (thank you!) pointing out that the way we resolve time fails as of Pandas 3, specifically here:

def resolve_time(t):
    try:
        return pd.to_datetime(t)          # Raises exception on "-2h" in Pandas <3, but returns Timestamp('1-01-01 02:00:00') in Pandas >=3
    except (TypeError, ValueError):
        pass
    return pd.to_datetime("now", utc=True) + pd.to_timedelta(t)

I think the simple fix Greg suggested of checking for a leading +/- is good and am adding that for string inputs.