fix timestampable use of datetimeclass #1577

Job	Run time
code-style-and-static-analysis	52s
Test Suite (7.4, sqlite, 5-min)	54s
Test Suite (7.4, sqlite, 5-max)	1m 19s
Test Suite (7.4, mysql, 5-min)	1m 13s
Test Suite (7.4, mysql, 5-max)	1m 38s
Test Suite (7.4, pgsql, 5-min)	1m 23s
Test Suite (7.4, pgsql, 5-max)	1m 48s
Test Suite (7.4, agnostic, 5-min)	29s
Test Suite (7.4, agnostic, 5-max)	1m 1s
Test Suite (8.2, sqlite, 5-min)	51s
Test Suite (8.2, sqlite, 5-max)	1m 8s
Test Suite (8.2, sqlite, 6-min)	1m 3s
Test Suite (8.2, sqlite, 6-max)	50s
Test Suite (8.2, sqlite, 7-min)	57s
Test Suite (8.2, sqlite, 7-max)	47s
Test Suite (8.2, mysql, 5-min)	38s
Test Suite (8.2, mysql, 5-max)	32s
Test Suite (8.2, mysql, 6-min)	46s
Test Suite (8.2, mysql, 6-max)	33s
Test Suite (8.2, mysql, 7-min)	31s
Test Suite (8.2, mysql, 7-max)	57s
Test Suite (8.2, pgsql, 5-min)	1m 10s
Test Suite (8.2, pgsql, 5-max)	1m 19s
Test Suite (8.2, pgsql, 6-min)	1m 12s
Test Suite (8.2, pgsql, 6-max)	1m 19s
Test Suite (8.2, pgsql, 7-min)	1m 10s
Test Suite (8.2, pgsql, 7-max)	1m 13s
Test Suite (8.2, agnostic, 5-min)	26s
Test Suite (8.2, agnostic, 5-max)	32s
Test Suite (8.2, agnostic, 6-min)	23s
Test Suite (8.2, agnostic, 6-max)	26s
Test Suite (8.2, agnostic, 7-min)	25s
Test Suite (8.2, agnostic, 7-max)	32s
	30m 17s

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