一、最终效果图
当前时间为2020-06-10 09:28:51
默认值为空

二、系统组件datetime.js源码
路径：node_modules\ionic-angular\components\datetime\datetime.js

/*** @hidden*/DateTime.prototype.generate = function () {var _this = this;// if a picker format wasn't provided, then fallback// to use the display formatvar template = this.pickerFormat || this.displayFormat || DEFAULT_FORMAT;if (isPresent(template)) {// make sure we've got up to date sizing informationthis.calcMinMax();// does not support selecting by day name// automaticallly remove any day name formatstemplate = template.replace('DDDD', '{~}').replace('DDD', '{~}');if (template.indexOf('D') === -1) {// there is not a day in the template// replace the day name with a numeric one if it existstemplate = template.replace('{~}', 'D');}// make sure no day name replacer is left in the stringtemplate = template.replace(/{~}/g, '');// parse apart the given template into an array of "formats"parseTemplate(template).forEach(function (format) {// loop through each format in the template// create a new picker column to build up with datavar key = convertFormatToKey(format);var values;// first see if they have exact values to use for this inputif (isPresent(_this[key + 'Values'])) {// user provide exact values for this date partvalues = convertToArrayOfNumbers(_this[key + 'Values'], key);}else {// use the default date part valuesvalues = dateValueRange(format, _this._min, _this._max);}var column = {name: key,selectedIndex: 0,options: values.map(function (val) {return {value: val,text: renderTextFormat(format, val, null, _this._locale),};})};// cool, we've loaded up the columns with options// preselect the option for this columnvar optValue = getValueFromFormat(_this.getValueOrDefault(), format);var selectedIndex = column.options.findIndex(function (opt) { return opt.value === optValue; });//于2020-06-10修改if(JSON.stringify(_this.value) == "{}"  && format == 'HH'){selectedIndex = selectedIndex + 8;}if (selectedIndex >= 0) {// set the select index for this column's optionscolumn.selectedIndex = selectedIndex;}// add our newly created column to the pickerpicker.addColumn(column);});// Normalize min/maxvar min_1 = this._min;var max_1 = this._max;var columns_1 = this._picker.getColumns();['month', 'day', 'hour', 'minute'].filter(function (name) { return !columns_1.find(function (column) { return column.name === name; }); }).forEach(function (name) {min_1[name] = 0;max_1[name] = 0;});this.divyColumns();}};

另一种不改源码的方法：
设置虚拟值,相当于设置了一个input 隐藏值，定时获取当前时间去初始化ion-datetime
{‘name’:‘计划开始时间’,‘index’:‘planstarttime’,‘type’:‘datetime’,‘placeholder’:‘请选择计划开始时间’,‘format’:‘YYYY-MM-DD HH:mm:ss’,‘disabled’:true,‘pickerHH’:true},

 <div class="itembox " *ngIf="item.type == 'datetime' "><div [class]="formVal[item.index] != undefined && formVal[item.index] != '' ? 'itembox datebox':'itembox'" ><ion-item><ion-label>{{item?.name}}<span *ngIf="item?.required" class="required">*</span></ion-label><ion-datetime *ngIf="formVal[item.index] == undefined || formVal[item.index] == ''" [displayFormat]="item.format" [name]="item.index"  [(ngModel)]="formVal[item.index +item.index]" [disabled] = "item.disabled" cancelText="取消"doneText="确认" (ionFocus)="ionFocusdate($event,item.index,i)" (ionCancel)= "ionCanceldate($event,item.index,format,i)" (ionChange)="ionChangedate($event,item.index,format,i)" style="opacity: 0;"></ion-datetime><ion-datetime *ngIf="formVal[item.index] != undefined && formVal[item.index] != ''" [displayFormat]="item.format" [name]="item.index" [placeholder]="item?.placeholder"  [(ngModel)]="formVal[item.index]" [disabled] = "item.disabled" cancelText="取消"doneText="确认" ></ion-datetime><span>22</span><ion-input type="text" *ngIf="formVal[item.index] == undefined || formVal[item.index] == ''"  [placeholder]="item?.placeholder"   [readonly]="true" style="position: absolute;display: inline;display: inline; width: auto;right: 0;top: 0;z-index: -1;"></ion-input></ion-item></div></div>

public foucsarray= {};
public timerarray={};
if(formdata[i].type == 'datetime'){this.foucsarray[formdata[i].index]=true;this.timerarray[formdata[i].index]=''; this.dateArray.push({'index':formdata[i].index,'format':formdata[i].format})ngAfterViewInit(){//初始化完组件视图及其子视图之后调用。var that = this; if(this.dateArray!=undefined){for(let i = 0;i<this.dateArray.length;i++){let datetimeindex = this.dateArray[i].index;let datetimeformat = this.dateArray[i].format;if(this.formVal[datetimeindex] == undefined || this.formVal[datetimeindex] == '' ){that.getint(datetimeindex)//   // }else{clearInterval(this.timerarray[datetimeindex]);}}}} }ngOnDestroy(){// console.log(331)if(this.dateArray!=undefined){for(let i = 0;i<this.dateArray.length;i++){let datetimeindex = this.dateArray[i].index;clearInterval(this.timerarray[datetimeindex]);}}
}
getint(index){var that = this;this.timerarray[index]= setInterval(function(){ that.formVal[index+index] =new Date(new Date().getTime()+8*60*60*1000).toISOString();},2000);
}
ionCanceldate(e,index,i){this.getint(index)this.foucsarray[index] = true;
}
ionFocusdate(e,index,i){this.foucsarray[index] = false;clearInterval(this.timerarray[index]);
}ionChangedate(e,index,format,i){let datetime =e.year + '-'+e.month +'-'+e.day +' '+e.hour+':'+e.minute+':'+e.second;if(clearInterval(this.timerarray[index]) == undefined && !this.foucsarray[index]){this.formVal[index] =new Date(new Date(datetime).getTime()+8*60*60*1000).toISOString();}}