fix timestampable use of datetimeclass #1585

Job	Run time
code-style-and-static-analysis	1m 14s
Test Suite (7.4, sqlite, 5-min)	53s
Test Suite (7.4, sqlite, 5-max)	1m 21s
Test Suite (7.4, mysql, 5-min)	1m 12s
Test Suite (7.4, mysql, 5-max)	1m 46s
Test Suite (7.4, pgsql, 5-min)	1m 12s
Test Suite (7.4, pgsql, 5-max)	1m 38s
Test Suite (7.4, agnostic, 5-min)	35s
Test Suite (7.4, agnostic, 5-max)	59s
Test Suite (8.2, sqlite, 5-min)	49s
Test Suite (8.2, sqlite, 5-max)	48s
Test Suite (8.2, sqlite, 6-min)	1m 26s
Test Suite (8.2, sqlite, 6-max)	50s
Test Suite (8.2, sqlite, 7-min)	1m 0s
Test Suite (8.2, sqlite, 7-max)	50s
Test Suite (8.2, mysql, 5-min)	30s
Test Suite (8.2, mysql, 5-max)	33s
Test Suite (8.2, mysql, 6-min)	31s
Test Suite (8.2, mysql, 6-max)	33s
Test Suite (8.2, mysql, 7-min)	31s
Test Suite (8.2, mysql, 7-max)	32s
Test Suite (8.2, pgsql, 5-min)	1m 12s
Test Suite (8.2, pgsql, 5-max)	1m 23s
Test Suite (8.2, pgsql, 6-min)	1m 21s
Test Suite (8.2, pgsql, 6-max)	1m 14s
Test Suite (8.2, pgsql, 7-min)	1m 49s
Test Suite (8.2, pgsql, 7-max)	1m 29s
Test Suite (8.2, agnostic, 5-min)	33s
Test Suite (8.2, agnostic, 5-max)	26s
Test Suite (8.2, agnostic, 6-min)	24s
Test Suite (8.2, agnostic, 6-max)	31s
Test Suite (8.2, agnostic, 7-min)	53s
Test Suite (8.2, agnostic, 7-max)	23s
	31m 21s

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