DiagrammeR Mermaid year 1990 shows w.00

Hello. I am trying to create a timeline with DiagrammeR Mermaid. I do not understand why year 1990 shows w.00 in the timeline.

library('DiagrammeR')
mermaid("
gantt
dateFormat  YYYY-MM-DD
title Sample Collections

section Collections1
Survey1:                                     first_1,    1966-01-01, 1972-12-31
Follow-up1:                                  first_2,    1973-01-01, 1973-12-31
Follow-up2:                                  first_3,    1978-01-01, 1980-12-31
Pilot-Study:                                 first_4,    2011-01-01, 2011-12-31
Study1:                                      first_5,    1992-01-01, 2012-12-31
Sub-Study:                                   first_6,    2007-01-01, 2007-12-31
Follow-Up:                                   first_7,    2014-01-01, 2014-12-31
Survey2:                                     first_8,    2000-01-01, 2000-12-31
Follow-Up:                                   first_9,    2011-01-01, 2011-12-31
Study2:                                      first_10,   2017-01-01, 2017-12-31

section Collections2
Study1:                       crit, done,    import_1,   1986-01-01, 2013-01-01
Ongoing Study1:               crit, active   import_2,   1990-01-01, 2021-12-31
Ongoing Study2:               crit, active   import_3,   1992-01-01, 2021-12-31
Collections2:                 crit, done,    import_4,   1994-01-01, 2008-12-31
Study:                        crit, done,    import_5,   2006-01-01, 2008-12-31
Study:                        crit, done,    import_6,   2015-01-01, 2018-12-31
Ongoing Study:                crit, active   import_7,   2017-01-01, 2021-12-31

section Other Collections
Study:                                        extras_1,   1981-01-01, 1990-12-31
Study:                                        extras_2,   1984-01-01, 1993-12-31
Study:                                        extras_3,   2015-01-01, 2018-12-31
Cohort:                                       extras_4,   1999-01-01, 2012-12-31
Study:                                        extras_5,   1994-01-01, 2021-12-31
Follow_up:                                    extras_6,   1991-01-01, 1995-12-31

")

Best wishes, Anitra