When creating a new contact, or updating an existing one that has a DATE type custom field, everything works fine in XML and REST V1 and V2 as long as the app has US style dates: month/day/year.
When the app is using non-US style dates (day/month/year), it works fine on XML and REST v1. On V2, it flips the day and month (if it can be flipped), and stores the incorrect date.
Example:
POST /v2/contacts
{“email_addresses”:[{“email”:”hello@plusthis.com”,”field”:”EMAIL1”}],”given_name”:”Hello”,”family_name”:”PlusThis”,”custom_fields”:[{“id”:75,”content”:”2027-02-10”}}}
The contact is created and the custom field is set to 2027-10-02
Full response:
{
"id":"82135",
"family_name":"plusthis",
"given_name":"hello",
"preferred_locale":null,
"addresses":[
],
"notes":null,
"owner_id":null,
"prefix":null,
"birth_date":null,
"origin":null,
"suffix":null,
"utm_parameters":[
],
"social_accounts":[
],
"contact_type":null,
"spouse_name":null,
"update_time":"2026-02-10T17:01:58.691Z",
"company":{
"id":null,
"company_name":""
},
"leadsource_id":null,
"links":[
],
"job_title":null,
"anniversary_date":null,
"email_addresses":[
{
"email":"hello@plusthis.com",
"field":"EMAIL1",
"opt_in_reason":null,
"is_opt_in":false,
"email_opt_status":"NON_MARKETABLE"
}
],
"tag_ids":[
],
"website":null,
"create_time":"2026-02-10T17:01:59.000Z",
"custom_fields":[
{
"id":"52",
"content":null
},
{
"id":"54",
"content":null
},
{
"id":"56",
"content":null
},
{
"id":"69",
"content":null
},
{
"id":"65",
"content":"None"
},
{
"id":"131",
"content":null
},
{
"id":"144",
"content":null
},
{
"id":"142",
"content":null
},
{
"id":"129",
"content":null
},
{
"id":"146",
"content":null
},
{
"id":"148",
"content":null
},
{
"id":"150",
"content":null
},
{
"id":"152",
"content":null
},
{
"id":"154",
"content":null
},
{
"id":"156",
"content":null
},
{
"id":"158",
"content":null
},
{
"id":"160",
"content":null
},
{
"id":"75",
"content":"2027-10-02T04:00:00.000Z"
},
{
"id":"77",
"content":null
},
{
"id":"79",
"content":null
},
{
"id":"81",
"content":null
},
{
"id":"83",
"content":null
},
{
"id":"85",
"content":null
},
{
"id":"87",
"content":null
},
{
"id":"89",
"content":null
},
{
"id":"119",
"content":null
},
{
"id":"121",
"content":null
},
{
"id":"51",
"content":null
},
{
"id":"127",
"content":null
},
{
"id":"133",
"content":null
},
{
"id":"125",
"content":null
},
{
"id":"135",
"content":null
},
{
"id":"137",
"content":null
},
{
"id":"139",
"content":null
}
],
"source_type":"API",
"middle_name":null,
"time_zone":null,
"fax_numbers":[
],
"phone_numbers":[
],
"referral_code":null,
"score_value":"0",
"preferred_name":null
}
PATCH /v2/contacts/82135
{“custom_fields”:[{“id”:75,”content”:”2027-02-10”}]}
The custom field is updated to 2027-10-02.
If we do the full ISO 8601 string, it is the same result
{“custom_fields”:[{“id”:75,”content”:”2027-02-10T05:00:00.000Z”}]}
The custom field is updated to 2027-10-02.
If the month and day are not flippable, it works as it should:
{“custom_fields”:[{“id”:75,”content”:”2027-02-15”}]}
This is stored as the correct date: 2027-02-15.
If this could be prioritized and resolved, we would appreciate it. We are having to fall back to REST v1 across many of our apps and tools that offer the ability to store dates due to this bug.