fix timestampable use of datetimeclass #1576

Job	Run time
code-style-and-static-analysis	1m 8s
Test Suite (7.4, sqlite, 5-min)	51s
Test Suite (7.4, sqlite, 5-max)	1m 24s
Test Suite (7.4, mysql, 5-min)	1m 27s
Test Suite (7.4, mysql, 5-max)	1m 32s
Test Suite (7.4, pgsql, 5-min)	1m 18s
Test Suite (7.4, pgsql, 5-max)	1m 36s
Test Suite (7.4, agnostic, 5-min)	36s
Test Suite (7.4, agnostic, 5-max)	1m 1s
Test Suite (8.2, sqlite, 5-min)	47s
Test Suite (8.2, sqlite, 5-max)	1m 9s
Test Suite (8.2, sqlite, 6-min)	48s
Test Suite (8.2, sqlite, 6-max)	49s
Test Suite (8.2, sqlite, 7-min)	58s
Test Suite (8.2, sqlite, 7-max)	57s
Test Suite (8.2, mysql, 5-min)	30s
Test Suite (8.2, mysql, 5-max)	46s
Test Suite (8.2, mysql, 6-min)	33s
Test Suite (8.2, mysql, 6-max)	30s
Test Suite (8.2, mysql, 7-min)	33s
Test Suite (8.2, mysql, 7-max)	32s
Test Suite (8.2, pgsql, 5-min)	1m 8s
Test Suite (8.2, pgsql, 5-max)	1m 19s
Test Suite (8.2, pgsql, 6-min)	1m 19s
Test Suite (8.2, pgsql, 6-max)	1m 13s
Test Suite (8.2, pgsql, 7-min)	1m 9s
Test Suite (8.2, pgsql, 7-max)	1m 9s
Test Suite (8.2, agnostic, 5-min)	40s
Test Suite (8.2, agnostic, 5-max)	31s
Test Suite (8.2, agnostic, 6-min)	2m 35s
Test Suite (8.2, agnostic, 6-max)	24s
Test Suite (8.2, agnostic, 7-min)	37s
Test Suite (8.2, agnostic, 7-max)	25s
	32m 14s

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