[BUG] cudf.to_datetime with format allows date strings with days beyond month limit

[mroeschke]

Describe the bug
cudf.to_datetime with format allows date strings with dates beyond month limit

Steps/Code to reproduce bug

In [7]: cudf.to_datetime("2015-02-99", format="%Y-%m-%d")
Out[7]: numpy.datetime64('2015-05-10T00:00:00.000000000')

In [8]: cudf.to_datetime("2015-02-99")
ValueError: Given date string not likely a datetime.  # OK

Expected behavior
Ideally a ValueError should be raised for Out[7]:

Environment overview (please complete the following information)

• Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
• Method of cuDF install: [conda, Docker, or from source]
  • If method of install is [Docker], provide docker pull & docker run commands used

Environment details
Please run and paste the output of the cudf/print_env.sh script here, to gather any other relevant environment details

Additional context
Add any other context about the problem here.

[mroeschke]

Looks like validation of the parsed string should occur here

cudf/cpp/src/strings/convert/convert_datetime.cu

Line 243 in a97020f

// special logic for each specifier

[davidwendt]

Validation is not done on data in general. Call cudf::strings::is_timestamp() if validation is required.

[mroeschke]

is_timestamp is what I'm looking for, thanks!