Plotted entries from OPSLOG files in VIMOS report BIASLEV, plot 6 (data set parQ4lo):
(DATE changes at 21:00 UT or MJD-OBS = xxx.875)
DATE       	MJD-OBS      	values
====================================================
2013-05-15	56428.435004440000000	206
2013-05-16	56429.368877970000000	206
2013-05-17	56430.368506550000000	206
2013-05-18	56431.269702390000000	205