improve handling of `since` option #992

Job	Run time
Pydantic V1	20s
Compatibility	30s
ubuntu-latest - python 3.8 - Lint	31s
ubuntu-latest - python 3.8 - Type check	29s
Docker	19s
ubuntu-latest - python 3.8 - Unit tests	1m 0s
ubuntu-latest - python 3.8 - Doc tests	1m 53s
ubuntu-latest - python 3.8 - Doc tests (B)	1m 37s
ubuntu-latest - python 3.8 - Images	48s
ubuntu-latest - python 3.8 - Jobs	19s
ubuntu-latest - python 3.8 - Experiments	32s
ubuntu-latest - python 3.8 - Datasets	1m 6s
ubuntu-latest - python 3.8 - Sweep example	48s
ubuntu-latest - python 3.8 - Build	29s
ubuntu-latest - python 3.8 - Style	23s
ubuntu-latest - python 3.8 - Docs	29s
Release	0s
	11m 33s

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